Semiconductor device and method for manufacturing semiconductor device

ABSTRACT

Disclosed is a semiconductor device including an insulating layer, a source electrode and a drain electrode embedded in the insulating layer, an oxide semiconductor layer in contact with the insulating layer, the source electrode, and the drain electrode, a gate insulating layer covering the oxide semiconductor layer, and a gate electrode over the gate insulating layer. The upper surface of the surface of the insulating layer, which is in contact with the oxide semiconductor layer, has a root-mean-square (RMS) roughness of 1 nm or less. There is a difference in height between an upper surface of the insulating layer and each of an upper surface of the source electrode and an upper surface of the drain electrode. The difference in height is preferably 5 nm or more. This structure contributes to the suppression of defects of the semiconductor device and enables their miniaturization.

TECHNICAL FIELD

The present invention relates to a semiconductor device and amanufacturing method thereof. Note that semiconductor devices hereinrefer to general elements and devices which function by utilizingsemiconductor characteristics.

BACKGROUND ART

There are a wide variety of metal oxides and such metal oxides are usedfor various applications. Indium oxide is a well-known material and isused as a material for transparent electrodes which are needed forliquid crystal display devices or the like.

Some metal oxides have semiconductor characteristics. Examples of suchmetal oxides having semiconductor characteristics include tungstenoxide, tin oxide, indium oxide, zinc oxide, and the like. A thin filmtransistor in which a channel formation region is formed using such ametal oxide has been already known (see, for example, Patent Documents 1to 4, Non-Patent Document 1, and the like).

Examples of metal oxides include not only a single-component oxide butalso a multi-component oxide. For example, InGaO₃(ZnO)_(m) (m: naturalnumber) having a homologous phase is known as a multi-component oxidesemiconductor including In, Ga, and Zn (see, for example, Non-PatentDocuments 2 to 4 and the like).

Furthermore, it has been confirmed that an oxide semiconductor includingsuch an In—Ga—Zn-based oxide can also be applied to a channel formationregion of a thin film transistor (see, for example, Patent Document 5,Non-Patent Documents 5 and 6, and the like).

In order to achieve high speed operation of a transistor or the like,miniaturization of the transistor is needed. For example, in PatentDocument 6, a thin film transistor including an oxide semiconductor usedfor a channel layer with a thickness of about 10 nm or smaller isdisclosed. In Non-Patent Document 7, a thin film transistor including anoxide semiconductor whose channel length is 2 μm to 100 μm is disclosed.

REFERENCES Patent Documents

-   [Patent Document 1] Japanese Published Patent Application No.    S60-198861-   [Patent Document 2] Japanese Published Patent Application No.    H8-264794-   [Patent Document 3] Japanese Translation of PCT International    Application No. H11-505377-   [Patent Document 4] Japanese Published Patent Application No.    2000-150900-   [Patent Document 5] Japanese Published Patent Application No.    2004-103957-   [Patent Document 6] Japanese Published Patent Application No.    2010-21170

Non-Patent Documents

-   [Non-Patent Document 1] M. W. Prins, K. O. Grosse-Holz, G.    Muller, J. F. M. Cillessen, J. B. Giesbers, R. P. Weening, and R. M.    Wolf, “A ferroelectric transparent thin-film transistor”, Appl.    Phys. Lett., 17 Jun. 1996, Vol. 68, pp. 3650-3652-   [Non-Patent Document 2] M. Nakamura, N. Kimizuka, and T. Mohri, “The    Phase Relations in the In₂O₃—Ga₂ZnO₄—ZnO System at 1350 ° C.”, J.    Solid State Chem., 1991, Vol. 93, pp. 298-315-   [Non-Patent Document 3] N. Kimizuka, M. Isobe, and M. Nakamura,    “Syntheses and Single-Crystal Data of Homologous Compounds,    In₂O₃(ZnO)_(m) (m=3, 4, and 5), InGaO₃(ZnO)₃, and Ga₂O₃(ZnO)_(m)    (m=7, 8, 9, and 16) in the In₂O₃—ZnGa₂O₄—ZnO System”, J. Solid State    Chem., 1995, Vol. 116, pp. 170-178-   [Non-Patent Document 4] M. Nakamura, N. Kimizuka, T. Mohri, and M.    Isobe, “Syntheses and crystal structures of new homologous    compounds, indium iron zinc oxides (InFeO₃(ZnO)_(m)) (m: natural    number) and related compounds”, KOTAI BUTSURI (SOLID STATE PHYSICS),    1993, Vol. 28, No. 5, pp. 317-327-   [Non-Patent Document 5] K. Nomura, H. Ohta, K. Ueda, T. Kamiya, M.    Hirano, and H. Hosono, “Thin-film transistor fabricated in    single-crystalline transparent oxide semiconductor”, SCIENCE, 2003,    Vol. 300, pp. 1269-1272-   [Non-Patent Document 6] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M.    Hirano, and H. Hosono, “Room-temperature fabrication of transparent    flexible thin-film transistors using amorphous oxide    semiconductors”, NATURE, 2004, Vol. 432, pp. 488-492-   [Non-Patent Document 7] T. Kawamura, H. Uchiyama, S. Saito, H.    Wakana, T. Mine, and M. Hatano, “Low-Voltage Operating Amorphous    Oxide TFTs”, IDW'09, pp. 1689-1692

DISCLOSURE OF INVENTION

In the case where a transistor is miniaturized, a defect generated inthe manufacturing process becomes a major problem. For example, in atransistor where a semiconductor layer is formed over a wiringfunctioning as a source or drain electrode, a gate electrode or thelike, the wiring has a larger thickness than the semiconductor layer,which causes poor coverage with the semiconductor layer when thethickness of the semiconductor layer is reduced along withminiaturization. As a result, disconnection, defective connection, orthe like may occur.

In the case where a transistor is miniaturized, another problem of ashort channel effect arises. The short-channel effect refers todegradation of electrical characteristics which becomes obvious withminiaturization of a transistor (a reduction in channel length (L)). Theshort-channel effect results from the effect of an electric field of adrain on a source. Specific examples of the short-channel effect are adecrease in threshold voltage, an increase in S value (subthresholdswing), an increase in leakage current, and the like. The short-channeleffect is likely to occur in a transistor including an oxidesemiconductor particularly because such a transistor cannot controlthreshold voltage by doping, unlike a transistor including silicon.

In view of this, it is an object of one embodiment of the disclosedinvention to provide a semiconductor device which suppresses a defectand achieves miniaturization. Further, it is another object of oneembodiment of the disclosed invention to provide a semiconductor devicewhich maintains favorable characteristics and achieves miniaturization.

An embodiment of the disclosed invention is a semiconductor deviceincluding an insulating layer, a source electrode and a drain electrodeembedded in the insulating layer, an oxide semiconductor layer incontact with a part of a surface of the insulating layer, a part of asurface of the source electrode, and a part of a surface of the drainelectrode, a gate insulating layer covering the oxide semiconductorlayer, and a gate electrode over the gate insulating layer. The part ofthe surface of the insulating layer, which is in contact with the oxidesemiconductor layer, has a root-mean-square (RMS) roughness of 1 nm orless. The difference in height between the part of the surface of theinsulating layer and the surface of the source electrode and thedifference in height between the part of the surface of the insulatinglayer and the surface of the drain electrode are 5 nm or more. Note thatin the above semiconductor device, the difference in height between thepart of the surface of the insulating layer and the surface of thesource electrode and the difference in height between the part of thesurface of the insulating layer and the surface of the drain electrodemay be 20 nm or less.

Note that in this specification, the root-mean-square (RMS) roughness isobtained by three-dimensionally expanding the RMS roughness of a crosssection curve so as to be able to apply it to the measurement surface.The RMS roughness is represented by the square root of the mean value ofthe square of the deviation from the reference surface to the specificsurface, and is obtained by the following formula.

$\begin{matrix}{R_{ms} = \sqrt{\frac{1}{S_{0}}{\int_{Y_{1}}^{Y_{2}}{\int_{X_{1}}^{X_{2}}{\{ {{F( {X,Y} )} - Z_{0}} \}^{2}{X}{Y}}}}}} & \lbrack {{Formula}\mspace{14mu} 1} \rbrack\end{matrix}$

Note that the measurement surface is a surface which is shown by all themeasurement data, and is represented by the following formula.

Z═F(X,Y)  [Formula 2]

The specific surface is a surface which is an object of roughnessmeasurement, and is a rectangular region which is surrounded by fourpoints represented by the coordinates (X₁, Y₁), (X₁, Y₂), (X₂, Y₁), and(X₂, Y₂). The area of the specific surface when the specific surface isflat ideally is denoted by S₀. Note that S₀ can be obtained by thefollowing formula.

S ₀ =|X ₂ −X ₁ |·|Y ₂ −Y ₁|  [Formula 3]

In addition, the reference surface refers to a surface parallel to anX-Y surface at the average height of the specific surface. In short,when the average value of the height of the specific surface is denotedby Z₀, the height of the reference surface is also denoted by Z₀. Notethat Z₀ can be obtained by the following formula.

$\begin{matrix}{Z_{0} = {\frac{1}{S_{0}}{\int_{Y_{1}}^{Y_{2}}{\int_{X_{1}}^{X_{2}}{{F( {X,Y} )}{X}{Y}}}}}} & \lbrack {{Formula}\mspace{14mu} 4} \rbrack\end{matrix}$

Note that in this specification, the root-mean-square (RMS) roughness iscalculated in a region of 10 nm×10 nm, preferably 100 nm×100 nm, morepreferably 1 μm×1 μm from an AFM image obtained using an atomic forcemicroscope (AFM).

Another embodiment of the disclosed invention is a semiconductor deviceincluding a first transistor, and a second transistor over the firsttransistor. The first transistor includes a first channel formationregion, a first gate insulating layer provided over the first channelformation region, a first gate electrode provided over the first gateinsulating layer so as to overlap with the first channel formationregion, and a first source electrode and a first drain electrodeelectrically connected to the first channel formation region. The secondtransistor includes a second source electrode and a second drainelectrode embedded in an insulating layer, a second channel formationregion in contact with a part of a surface of the insulating layer, apart of a surface of the second source electrode, and a part of asurface of the second drain electrode, a second gate insulating layercovering the second channel formation region, and a second gateelectrode over the second gate insulating layer. The part of the surfaceof the insulating layer, which is in contact with the second channelformation region, has a root-mean-square roughness of 1 nm or less. Thedifference in height between the part of the surface of the insulatinglayer and the surface of the second source electrode and the differencein height between the part of the surface of the insulating layer andthe surface of the second drain electrode are 5 nm or more. Note that inthe above semiconductor device, the difference in height between thepart of the surface of the insulating layer and the surface of thesecond source electrode and the difference in height between the part ofthe surface of the insulating layer and the surface of the second drainelectrode may be 20 nm or less.

Another embodiment of the disclosed invention is a method formanufacturing a semiconductor device, including the steps of: forming asource electrode and a drain electrode over a surface with aroot-mean-square roughness of 1 nm or less; forming an insulating layerso as to cover the source electrode and the drain electrode; performingplanarization treatment of a surface of the insulating layer, therebyforming a planarized insulating layer having a surface with aroot-mean-square roughness of 1 nm or less; forming an opening reachingthe source electrode and the drain electrode in the planarizedinsulating layer, so that the difference in height between a part of thesurface of the planarized insulating layer and a surface of the sourceelectrode and the difference in height between the part of the surfaceof the planarized insulating layer and a surface of the drain electrodeare 5 nm or more; forming an oxide semiconductor layer in contact withthe part of the surface of the planarized insulating layer, a part ofthe surface of the source electrode, and a part of the surface of thedrain electrode; forming a gate insulating layer so as to cover theoxide semiconductor layer; and forming a gate electrode over the gateinsulating layer.

Another embodiment is a method for manufacturing a semiconductor device,including the steps of: forming a source electrode and a drain electrodeover a surface with a root-mean-square roughness of 1 nm or less;forming an insulating layer so as to cover the source electrode and thedrain electrode; performing planarization treatment of a surface of theinsulating layer, thereby forming a planarized insulating layer having asurface with a root-mean-square roughness of 1 nm or less and exposingthe source electrode and the drain electrode; thinning the sourceelectrode and the drain electrode so that the difference in heightbetween a part of the surface of the planarized insulating layer and asurface of the source electrode and the difference in height between thepart of the surface of the planarized insulating layer and a surface ofthe drain electrode are 5 nm or more; forming an oxide semiconductorlayer in contact with the part of the surface of the planarizedinsulating layer, a part of the surface of the source electrode, and apart of the surface of the drain electrode; forming a gate insulatinglayer so as to cover the oxide semiconductor layer; and forming a gateelectrode over the gate insulating layer. Note that in the above methodfor manufacturing a semiconductor device, the difference in heightbetween the part of the surface of the planarized insulating layer andthe surface of the source electrode and the difference in height betweenthe part of the surface of the planarized insulating layer and thesurface of the drain electrode may be 20 nm or less.

Another embodiment of the disclosed invention is a method formanufacturing a semiconductor device, including the steps of: forming afirst transistor including a channel formation region, a first gateinsulating layer over the channel formation region, a first gateelectrode over the first gate insulating layer, overlapping with thechannel formation region, and a first source electrode and a first drainelectrode electrically connected to the channel formation region;forming a first insulating layer having a surface with aroot-mean-square roughness of 1 nm or less so as to cover the firsttransistor; forming a second source electrode and a second drainelectrode over the surface of the first insulating layer; forming asecond insulating layer so as to cover the second source electrode andthe second drain electrode; performing planarization treatment of asurface of the second insulating layer, thereby forming a planarizedinsulating layer having a surface with a root-mean-square roughness of 1nm or less; forming an opening reaching the second source electrode andthe second drain electrode in the planarized insulating layer, so thatthe difference in height between a part of the surface of the planarizedinsulating layer and a surface of the second source electrode and thedifference in height between the part of the surface of the planarizedinsulating layer and a surface of the second drain electrode are 5 nm ormore; forming an oxide semiconductor layer in contact with the part ofthe surface of the planarized insulating layer, a part of the surface ofthe second source electrode, and a part of the surface of the seconddrain electrode; forming a second gate insulating layer so as to coverthe oxide semiconductor layer; and forming a second gate electrode overthe second gate insulating layer.

Another embodiment is a method for manufacturing a semiconductor device,including the steps of: forming a first transistor including a channelformation region, a first gate insulating layer over the channelformation region, a first gate electrode over the first gate insulatinglayer, overlapping with the channel formation region, and a first sourceelectrode and a first drain electrode electrically connected to thechannel formation region; forming a first insulating layer having asurface with a root-mean-square roughness of 1 nm or less so as to coverthe first transistor; forming a second source electrode and a seconddrain electrode over the surface of the first insulating layer; forminga second insulating layer so as to cover the second source electrode andthe second drain electrode; performing planarization treatment of asurface of the second insulating layer, thereby forming a planarizedinsulating layer having a surface with a root-mean-square roughness of 1nm or less and exposing the second source electrode and the second drainelectrode; thinning the second source electrode and the second drainelectrode so that the difference in height between a part of the surfaceof the planarized insulating layer and a surface of the second sourceelectrode and the difference in height between the part of the surfaceof the planarized insulating layer and a surface of the second drainelectrode are 5 nm or more; forming an oxide semiconductor layer incontact with the part of the surface of the planarized insulating layer,a part of the surface of the second source electrode, and a part of thesurface of the second drain electrode; forming a second gate insulatinglayer so as to cover the oxide semiconductor layer; and forming a secondgate electrode over the second gate insulating layer. Note that in theabove method for manufacturing a semiconductor device, the difference inheight between the part of the surface of the planarized insulatinglayer and the surface of the second source electrode and the differencein height between the part of the surface of the planarized insulatinglayer and the surface of the second drain electrode may be 20 nm orless.

Note that the channel length L of the transistor is preferably less than2 μm, further preferably, 10 nm to 350 nm (0.35 μm). The thickness ofthe oxide semiconductor layer is in the range of 1 nm to 50 nm,preferably, 2 nm to 20 nm, further preferably, 3 nm to 15 nm With such astructure, a semiconductor device which operates at high speed andconsumes less power can be achieved. For the gate insulating layer, ahigh dielectric constant material such as hafnium oxide is used. Forexample, the relative permittivity of hafnium oxide is approximately 15,which is much higher than that of silicon oxide which is 3 to 4. Withsuch a material, a gate insulating layer where the equivalent oxidethickness is less than 15 nm, preferably 2 nm to 10 nm, can be easilyformed. In other words, the semiconductor device can be easilyminiaturized. Further, as the oxide semiconductor layer, an intrinsicoxide semiconductor which is purified is used. With such an oxidesemiconductor, the carrier density of the oxide semiconductor layer canbe, for example, less than 1×10¹²/cm³, preferably, less than1.45×10¹⁰/cm³, the off-state current of the transistor can be 100 zA/μm(1 zA (zeptoampere) is 1×10⁻²¹ A) or less, preferably, 10 zA/μm or less,and the S value of the transistor can be 65 mV/dec or less, preferably,less than 63 mV/dec. When the above structure is employed, the off-statecurrent of the transistor can be 1×10⁻²⁴ A/μm to 1×10⁻³⁰ A/λm in theory.The gate electrode may be provided to overlap with the source electrodeand the drain electrode, and alternatively, only an end portion of thegate electrode may be provided to overlap with an end portion of thesource electrode and an end portion of the drain electrode.

Note that semiconductor devices herein refer to general devices whichfunction by utilizing semiconductor characteristics. For example, adisplay device, a memory device, an integrated circuit, and the like areincluded in the category of the semiconductor device.

Note that the term such as “over” or “below” in this specification doesnot necessarily mean that a component is placed “directly on” or“directly under” another component. For example, the expression “a gateelectrode over a gate insulating layer” does not exclude the case wherea component is placed between the gate insulating layer and the gateelectrode.

In addition, the term such as “electrode” or “wiring” in thisspecification does not limit a function of a component. For example, an“electrode” can be used as part of a “wiring”, and the “wiring” can beused as part of the “electrode”. Furthermore, the term “electrode” or“wiring” can include the case where a plurality of “electrodes” or“wirings” is formed in an integrated manner.

Functions of a “source” and a “drain” are sometimes interchanged witheach other when a transistor of opposite polarity is used or when thedirection of current flowing is changed in circuit operation, forexample. Therefore, the terms “source” and “drain” can be used to denotethe drain and the source, respectively, in this specification.

Note that the term “electrically connected” in this specificationincludes the case where components are connected through an “objecthaving any electric function”. There is no particular limitation on anobject having any electric function as long as electric signals can betransmitted and received between components that are connected throughthe object. Examples of an “object having any electric function” are aswitching element such as a transistor, a resistor, an inductor, acapacitor, and an element with a variety of functions, as well as anelectrode and a wiring.

According to one embodiment of the disclosed invention, a problem causedby miniaturization can be resolved. As a result, the size of thetransistor can be sufficiently reduced. By sufficiently downsizing thetransistor, an area of a semiconductor device including the transistoris decreased, and the number of semiconductor devices manufactured withone substrate is increased. Therefore, the manufacturing cost persemiconductor device is decreased. Further, since the semiconductordevice is downsized, a semiconductor device with a size similar to thatof the conventional semiconductor device can have improved functions.Moreover, advantageous effects such as high-speed operation and lowpower consumption due to a reduction in channel length can be obtained.That is, according to one embodiment of the disclosed invention,miniaturization of a transistor including an oxide semiconductor isachieved, so that a variety of advantageous effects accompanyingtherewith can be obtained.

As described above, according to one embodiment of the disclosedinvention, a semiconductor device which achieves miniaturization as wellas suppressing defect or maintaining favorable characteristics can beprovided.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A to 1C are cross-sectional views each illustrating an example ofa structure of a semiconductor device.

FIGS. 2A to 2G are cross-sectional views illustrating a manufacturingprocess of a semiconductor device.

FIGS. 3A to 3G are cross-sectional views illustrating a manufacturingprocess of a semiconductor device.

FIGS. 4A to 4C are a cross-sectional view, a plan view, and a circuitdiagram illustrating an example of a structure of a semiconductordevice.

FIGS. 5A to 5D are cross-sectional views illustrating a manufacturingprocess of a semiconductor device.

FIGS. 6A to 6C are cross-sectional views illustrating a manufacturingprocess of a semiconductor device.

FIGS. 7A-1, 7A-2, and 7B are diagrams illustrating an example ofapplication of a semiconductor device.

FIGS. 8A and 8B are diagrams illustrating an example of application of asemiconductor device.

FIGS. 9A to 9C are diagrams illustrating an example of application of asemiconductor device.

FIG. 10 is a diagram illustrating an example of application of asemiconductor device.

FIGS. 11A and 11B are diagrams illustrating an example of application ofa semiconductor device.

FIGS. 12A to 12F are diagrams each illustrating an electronic deviceincluding a semiconductor device.

FIGS. 13A and 13B are diagrams each illustrating a model used forcalculation.

FIG. 14 is a diagram showing the relationship between gate voltage VG(V) and drain current ID (A).

FIG. 15 is a diagram showing the relationship between channel length L(nm) and threshold voltage Vth (V).

FIG. 16 is a diagram showing the relationship between channel length L(nm) and S value (V/dec).

FIGS. 17A and 17B are diagrams each illustrating a model used forcalculation.

FIG. 18 is a diagram showing the relationship between gate voltage VG(V) and drain current ID (A).

BEST MODE FOR CARRYING OUT THE INVENTION

Examples of embodiments of the present invention will be described belowwith reference to the drawings. Note that the present invention is notlimited to the following description and it will be readily appreciatedby those skilled in the art that the modes and details of the presentinvention can be modified in various ways without departing from thespirit and scope thereof. Therefore, the present invention should not beinterpreted as being limited to the description in the followingembodiments.

Note that the position, size, range, or the like of each componentillustrated in drawings and the like is not accurately represented insome cases for easy understanding. Therefore, the disclosed invention isnot necessarily limited to the position, size, range, or the like asdisclosed in the drawings and the like.

Note that ordinal numbers such as “first”, “second”, and “third” in thisspecification are used in order to avoid confusion among components, andthe terms do not limit the components numerically.

Embodiment 1

In this embodiment, a structure and a manufacturing method of asemiconductor device according to one embodiment of the disclosedinvention will be described with reference to FIGS. 1A to 1C, FIGS. 2Ato 2G, and FIGS. 3A to 3G.

<Example of Structure of Semiconductor Device>

FIGS. 1A to 1C each illustrate an example of a structure of asemiconductor device. FIG. 1A is a first structural example, and FIG. 1Bis a second structural example. FIG. 1C is a modified example of FIG.1A.

A transistor 162 in FIG. 1A includes an insulating layer 143 b over asubstrate 140 having a surface where components are formed, a sourceelectrode 142 a and a drain electrode 142 b embedded in an insulatinglayer including the insulating layer 143 b, an oxide semiconductor layer144 in contact with part of an upper surface of the insulating layer 143b, an upper surface of the source electrode 142 a, and an upper surfaceof the drain electrode 142 b, a gate insulating layer 146 covering theoxide semiconductor layer 144, and a gate electrode 148 a over the gateinsulating layer 146.

With the use of an oxide semiconductor for an active layer of atransistor as illustrated in FIG. 1A, favorable characteristics can beobtained. For example, the S value of a transistor can be 65 mV/dec orless, preferably less than 63 mV/dec.

In addition, part of the upper surface of the insulating layer 143 b(particularly referring to a region parallel to the surface wherecomponents are formed), which is in contact with the oxide semiconductorlayer, has a root-mean-square (RMS) roughness of 1 nm or less(preferably 0.5 nm or less). The difference in height between the partof the upper surface of the insulating layer 143 b and the upper surfaceof the source electrode 142 a and the difference in height between thepart of the upper surface of the insulating layer 143 b and the uppersurface of the drain electrode 142 b are 5 nm or more.

As described above, in one embodiment of the disclosed invention, achannel formation region of the transistor 162 is provided over a highlyflat region with a root-mean-square (RMS) roughness of 1 nm or less.This makes it possible to prevent a problem such as a short-channeleffect even in a situation where the transistor 162 is miniaturized andto provide the transistor 162 with favorable characteristics.

In addition, the oxide semiconductor layer 144 can have a uniformthickness by improving the planarity of the surface where components areformed, and the transistor 162 can have improved characteristics.Furthermore, a decrease in coverage which may be caused by a largedifference in height can be suppressed, and a disconnection or adefective connection of the oxide semiconductor layer 144 can beprevented.

Moreover, the path of electric current can be extended by making a smalldifference in height (e.g., 5 nm to 20 nm) between part of the uppersurface of the insulating layer 143 b and the upper surface of thesource electrode 142 a and between the part of the upper surface of theinsulating layer 143 b and the upper surface of the drain electrode 142b. This makes it possible to alleviate the concentration of an electricfield in the transistor 162 and to suppress a short-channel effect.

Here, the oxide semiconductor layer 144 is preferably an oxidesemiconductor layer which is purified by sufficiently removing animpurity such as hydrogen therefrom or by sufficiently supplying oxygenthereto. Specifically, the hydrogen concentration of the oxidesemiconductor layer 144 is 5×10¹⁹ atoms/cm³ or less, preferably 5×10¹⁸atoms/cm³ or less, more preferably 5×10¹⁷ atoms/cm³ or less, forexample. Note that the above hydrogen concentration of the oxidesemiconductor layer 144 is measured by secondary ion mass spectrometry(SIMS). The concentration of carriers generated due to a donor such ashydrogen in the oxide semiconductor layer 144, in which hydrogen isreduced to a sufficiently low concentration so that the oxidesemiconductor layer is purified and in which defect states in an energygap due to oxygen deficiency are reduced by sufficiently supplyingoxygen as described above, is less than 1×10¹²/cm³, preferably less than1×10¹¹/cm³, more preferably less than 1.45×10¹⁹/cm³. In addition, forexample, the off-state current (per unit channel width (1 μm), here) atroom temperature (25° C.) is 100 zA (1 zA (zeptoampere) is 1×10⁻²¹ A) orless, preferably 10 zA or less. In this manner, by using an i-type(intrinsic) or substantially i-type oxide semiconductor, the transistor162 which has extremely favorable off-state current characteristics canbe obtained.

Note that as disclosed in Non-Patent Document 7 and the like, arelatively large-size transistor whose channel length is 2 μm to 100 μmcan be manufactured with use of an n-type oxide semiconductor having ahigh carrier density of 2×10¹⁹/cm³. However, when such a material isapplied to a miniaturized transistor whose channel length is less than 2μm, the threshold voltage drastically shifts negatively, and thus it isdifficult to realize a normally-off transistor. In other words, thetransistor which has a channel length of less than 2 μm and ismanufactured using such a material does not work in practice. Incontrast, an intrinsic or substantially intrinsic oxide semiconductorwhich is purified has a carrier density of at most less than 1×10¹⁴/cm³,which does not cause a problem of normally on; thus, with use of such anintrinsic or substantially intrinsic oxide semiconductor, a transistorwhose channel length is less than 2 μm can be easily realized.

A transistor 262 in FIG. 1B has a structure similar to that of thetransistor 162. That is, the transistor 262 includes an insulating layer243 b over a substrate 240 having a surface where components are formed,a source electrode 242 a and a drain electrode 242 b embedded in aninsulating layer including the insulating layer 243 b, an oxidesemiconductor layer 244 in contact with part of an upper surface of theinsulating layer 243 b, an upper surface of the source electrode 242 a,and an upper surface of the drain electrode 242 b, a gate insulatinglayer 246 covering the oxide semiconductor layer 244, and a gateelectrode 248 a over the gate insulating layer 246.

In addition, part of the upper surface of the insulating layer 243 b,which is in contact with the oxide semiconductor layer, has aroot-mean-square (RMS) roughness of 1 nm or less (preferably 0.5 nm orless). The difference in height between the part of the upper surface ofthe insulating layer 243 b and the upper surface of the source electrode242 a and the difference in height between the part of the upper surfaceof the insulating layer 243 b and the upper surface of the drainelectrode 242 b are 5 nm or more.

A difference between the transistor 262 in FIG. 1B and the transistor162 in FIG. 1A is whether or not part of the insulating layer isprovided so as to cover end portions of the source electrode and thedrain electrode. In other words, the insulating layer 143 b of thetransistor 162 in FIG. 1A is provided so as to cover the sourceelectrode 142 a and the drain electrode 142 b, whereas the insulatinglayer 243 b of the transistor 262 in FIG. 1B does not cover the sourceelectrode 242 a and the drain electrode 242 b.

The effects resulting from the structure in FIG. 1B are similar to thoseobtained in the case of FIG. 1A. In other words, because a channelformation region of the transistor 262 is provided over a highly flatregion, a problem such as a short-channel effect can be prevented evenin a situation where the transistor 262 is miniaturized, and thetransistor 262 with favorable characteristics can be provided. Inaddition, the oxide semiconductor layer 244 can have a uniform thicknessby improving the planarity of the surface where components are formed,and the transistor 262 can have improved characteristics. Furthermore, adecrease in coverage which may be caused by a large difference in heightcan be suppressed, and a disconnection or a defective connection of theoxide semiconductor layer 244 can be prevented. Moreover, by making asmall difference in height between the insulating layer and the sourceelectrode (and the drain electrode) as described above, theconcentration of an electric field in the transistor 262 can bealleviated, and a short-channel effect can be suppressed.

A transistor 162 in FIG. 1C is a modified example of the transistor 162in FIG. 1A. Specifically, the transistor 162 in FIG. 1C corresponds to atransistor obtained by modifying the source electrode 142 a and thedrain electrode 142 b in FIG. 1A into a tapered shape. The taper anglecan be, for example, greater than or equal to 30° and less than or equalto 60°. Note that the “taper angle” means an angle formed by the sidesurface and the bottom surface of a layer having a tapered shape (forexample, the source electrode 142 a) when observed from a directionperpendicular to a cross section thereof (a plane perpendicular to asurface of the substrate 140).

<Example of Method for Manufacturing Semiconductor Device>

Next, an example of a method for manufacturing the semiconductor devicewill be described with reference to FIGS. 2A to 2G and FIGS. 3A to 3G.Here, FIGS. 2A to 2G illustrate an example of a method for manufacturingthe transistor 162 shown in FIG. 1A, and FIGS. 3A to 3G illustrate anexample of a method for manufacturing the transistor 262 shown in FIG.1B. Note that FIGS. 2A to 2G will first be described below, and then,for FIGS. 3A to 3G, only a main difference from FIGS. 2A to 2G will bedescribed. In addition, the structure shown in FIG. 1C can bemanufactured in a manner similar to that of the structure shown in FIG.1A; thus, the detailed description thereof is omitted.

FIGS. 2A to 2G will be described below. First, the source electrode 142a and the drain electrode 142 b are formed over the substrate 140 havinga surface where components are formed (see FIG. 2A).

Although there is no particular limitation on a substrate which can beused as the substrate 140, it is necessary that the substrate 140 has atleast heat resistance high enough to withstand heat treatment to beperformed later. For example, the substrate may be a glass substrate, aceramic substrate, a quartz substrate, a sapphire substrate, or thelike. Alternatively, the substrate may be a single crystal semiconductorsubstrate or a polycrystalline semiconductor substrate of silicon,silicon carbide, or the like, a compound semiconductor substrate ofsilicon germanium or the like, an SOI substrate, or the like as long asthe substrate has an insulating surface. Still alternatively, thesubstrate may be any of these substrates provided with a semiconductorelement. Still alternatively, the substrate 140 may be provided with abase film.

Note that the preferred surface of the substrate 140 where componentsare formed is a sufficiently flat surface. For example, a surface with aroot-mean-square roughness (RMS) of 1 nm or less (preferably 0.5 nm orless) is employed. When the transistor 162 is formed over such asurface, the characteristics can be sufficiently improved. In the casewhere the surface of the substrate 140 has poor flatness, it isdesirable that the surface be subjected to chemical mechanical polishing(CMP) treatment, etching treatment, or the like so as to have the aboveflatness. Note that, for the details of the CMP treatment, thedescription of CMP treatment for an insulating layer 143 mentioned belowcan be referred to.

The source electrode 142 a and the drain electrode 142 b can be formedby forming a conductive layer over the substrate 140 having a surfacewhere components are formed and then selectively etching the conductivelayer.

The above conductive layer can be formed by a PVD method such as asputtering method, or a CVD method such as a plasma CVD method. As amaterial of the conductive layer, an element selected from aluminum,chromium, copper, tantalum, titanium, molybdenum, and tungsten, an alloyincluding any of these elements as a component, or the like can be used.A material including one of manganese, magnesium, zirconium, beryllium,neodymium, or scandium or a combination of a plurality of these elementsmay be used.

The conductive layer may have a single-layer structure or astacked-layer structure including two or more layers. For example, theconductive layer may have a single-layer structure of a titanium film ora titanium nitride film, a single-layer structure of an aluminum filmincluding silicon, a two-layer structure in which a titanium film isstacked over an aluminum film, a two-layer structure in which a titaniumfilm is stacked over a titanium nitride film, a three-layer structure inwhich a titanium film, an aluminum film, and a titanium film are stackedin this order, or the like. Note that the conductive layer having asingle-layer structure of a titanium film or a titanium nitride film hasan advantage in that it can be easily processed into the sourceelectrode 142 a and the drain electrode 142 b having a tapered shape.

The conductive layer may be formed using a conductive metal oxide. Asthe conductive metal oxide, indium oxide (In₂O₃), tin oxide (SnO₂), zincoxide (ZnO), an indium oxide-tin oxide alloy (In₂O₃—SnO₂, which isabbreviated to ITO in some cases), an indium oxide-zinc oxide alloy(In₂O₃—ZnO), or any of these metal oxide materials including silicon orsilicon oxide can be used.

Although either dry etching or wet etching may be performed as theetching of the conductive layer, dry etching with high controllabilityis preferably used for miniaturization. The etching may be performed sothat the source electrode 142 a and the drain electrode 142 b to beformed have a tapered shape. The taper angle can be, for example,greater than or equal to 30° and less than or equal to 60°.

The channel length (L) of the transistor 162 is determined by a distancebetween upper edge portions of the source electrode 142 a and the drainelectrode 142 b. Note that for light exposure for forming a mask in thecase of manufacturing a transistor with a channel length (L) of lessthan 25 nm, light exposure is preferably performed with extremeultraviolet light whose wavelength is several nanometers to several tensof nanometers, which is extremely short. The resolution of lightexposure with extreme ultraviolet light is high and the depth of focusis large. For these reasons, the channel length (L) of the transistor tobe formed later can be set to less than 2 μm, preferably in the range of10 nm to 350 nm (0.35 μm), in which case the circuit can operate athigher speed. In addition, power consumption of the semiconductor devicecan be reduced by miniaturization.

Next, the insulating layer 143 is formed so as to cover the sourceelectrode 142 a and the drain electrode 142 b (see FIG. 2B).

The insulating layer 143 can be formed using an inorganic insulatingmaterial such as silicon oxide, silicon oxynitride, silicon nitride, oraluminum oxide. It is particularly preferable that the insulating layer143 be formed using silicon oxide because the oxide semiconductor layer144 formed later is in contact with the insulating layer 143. Althoughthere is no particular limitation on the forming method of theinsulating layer 143, in consideration of contact with the oxidesemiconductor layer 144, a method in which hydrogen is sufficientlyreduced is preferably employed. Examples of such a method include asputtering method and the like. Needless to say, another depositionmethod such as a plasma CVD method may be used.

Next, an insulating layer 143 a is formed by thinning the insulatinglayer 143 by chemical mechanical polishing (CMP) treatment (see FIG.2C). Here, the CMP treatment is performed under such conditions that thesurfaces of the source electrode 142 a and the drain electrode 142 bremain unexposed. In addition, the CMP treatment is performed under suchconditions that the root-mean-square (RMS) roughness of a surface of theinsulating layer 143 a becomes 1 nm or less (preferably 0.5 nm or less).By the CMP treatment performed under such conditions, the planarity of asurface where the oxide semiconductor layer 144 is formed later can beimproved, and the characteristics of the transistor 162 can be improved.

The CMP treatment is a method for planarizing a surface of an object tobe processed by a combination of chemical and mechanical actions. Morespecifically, the CMP treatment is a method in which a polishing clothis attached to a polishing stage, the polishing stage and an object tobe processed are each rotated or swung while a slurry (an abrasive) issupplied between the object to be processed and the polishing cloth, andthe surface of the object to be processed is polished by a chemicalreaction between the slurry and the surface of the object to beprocessed and by a mechanical polishing action of the polishing cloth onthe object to be processed.

Note that the CMP treatment may be performed only once or plural times.When the CMP treatment is performed plural times, it is preferable thatfirst polishing be performed at a high polishing rate and finalpolishing be performed at a low polishing rate. By performing polishingat different polishing rates, the planarity of the surface of theinsulating layer 143 a can be further improved.

Next, the insulating layer 143 b having openings reaching the sourceelectrode 142 a and the drain electrode 142 b is formed by selectivelyremoving regions of the insulating layer 143 a, which overlap with thesource electrode 142 a and the drain electrode 142 b (see FIG. 2D). Theinsulating layer 143 a can be selectively removed by a method such asetching.

The insulating layer 143 a may be etched by either dry etching or wetetching. Further, the insulating layer 143 a is preferably etched sothat the openings to be formed have an inclined surface. The inclinedsurface preferably has an angle formed with the surface of the substrate140 and the inclined surface greater than or equal to 30° and less thanor equal to 60° when seen from a direction perpendicular to a crosssection (a plane perpendicular to the surface of the substrate 140) ofthe inclined surface. By etching the insulating layer 143 a under suchconditions, coverage with the oxide semiconductor layer to be formedlater so as to cover the insulating layer 143 b is increased, so that adisconnection of the oxide semiconductor layer or the like can beprevented.

By the above etching step, a surface having a small difference in height(e.g., 5 nm to 20 nm) is formed by part of an upper surface of theinsulating layer 143 b, an upper surface of the source electrode 142 a,and an upper surface of the drain electrode 142 b. In addition, endportions of the source electrode 142 a and the drain electrode 142 b arecovered with the insulating layer 143 b. Note that, with a higherpatterning accuracy in forming the openings, it is possible to obtain astructure in which end portions of the source electrode 142 a and thedrain electrode 142 b are not covered with the insulating layer 143 b.

Next, the oxide semiconductor layer 144 covering the above-describedsurface is formed in contact with part of the source electrode 142 a,the drain electrode 142 b, and the insulating layer 143 b; then, thegate insulating layer 146 is formed so as to cover the oxidesemiconductor layer 144 (see FIG. 2E).

The oxide semiconductor layer 144 contains at least one element selectedfrom In, Ga, Sn, and Zn. For example, a four-component metal oxide suchas an In—Sn—Ga—Zn—O-based oxide semiconductor, a three-component metaloxide such as an In—Ga—Zn—O-based oxide semiconductor, anIn—Sn—Zn—O-based oxide semiconductor, an In—Al—Zn—O-based oxidesemiconductor, a Sn—Ga—Zn—O-based oxide semiconductor, anAl—Ga—Zn—O-based oxide semiconductor, or a Sn—Al—Zn—O-based oxidesemiconductor, a two-component metal oxide such as an In—Zn—O-basedoxide semiconductor, a Sn—Zn—O-based oxide semiconductor, anAl—Zn—O-based oxide semiconductor, a Zn—Mg—O-based oxide semiconductor,a Sn—Mg—O-based oxide semiconductor, an In—Mg—O-based oxidesemiconductor, or an In—Ga—O-based oxide semiconductor, asingle-component metal oxide such as an In—O-based oxide semiconductor,a Sn—O-based oxide semiconductor, or a Zn—O-based oxide semiconductor,or the like can be used. In addition, any of the above oxidesemiconductors may contain an element other than In, Ga, Sn, and Zn, forexample, SiO₂.

For example, an In—Ga—Zn—O-based oxide semiconductor means an oxide filmcontaining indium (In), gallium (Ga), and zinc (Zn), and there is nolimitation on the composition ratio thereof.

In particular, an In—Ga—Zn—O-based oxide semiconductor material hassufficiently high resistance when there is no electric field and thusoff-state current can be sufficiently reduced. In addition, also havinghigh field-effect mobility, the In—Ga—Zn—O-based oxide semiconductormaterial is suitable for a semiconductor material used in asemiconductor device.

As a typical example of the In—Ga—Zn—O-based oxide semiconductormaterial, an oxide semiconductor material represented by InGaO₃(ZnO),(m>0) is given. Using M instead of Ga, there is an oxide semiconductormaterial represented by InMO₃(ZnO)_(m) (m>0). Here, M denotes one ormore metal elements selected from gallium (Ga), aluminum (Al), iron(Fe), nickel (Ni), manganese (Mn), cobalt (Co), or the like. Forexample, M may be Ga, Ga and Al, Ga and Fe, Ga and Ni, Ga and Mn, Ga andCo, or the like. Note that the above-described compositions are derivedfrom the crystal structures that the oxide semiconductor material canhave, and are mere examples.

In the case where an In—Zn—O-based material is used as an oxidesemiconductor, a target therefore has a composition ratio of In:Zn=50:1to 1:2 in an atomic ratio (In₂O₃:ZnO=25:1 to 1:4 in a molar ratio),preferably, In:Zn=20:1 to 1:1 in an atomic ratio (In₂O₃:ZnO=10:1 to 1:2in a molar ratio), further preferably, In:Zn=15:1 to 1.5:1 in an atomicratio (In₂O₃:ZnO=15:2 to 3:4 in a molar ratio). For example, in a targetused for formation of an In—Zn—O-based oxide semiconductor which has anatomic ratio of In:Zn:O═X:Y:Z, the relation of Z>1.5X+Y is satisfied.

As a target used for forming the oxide semiconductor layer 144 by asputtering method, a target having a composition ratio of In:Ga:Zn=1:x:y(x is greater than or equal to 0 and y is greater than or equal to 0.5and less than or equal to 5) is preferably used. For example, a targethaving a composition ratio of In₂O₃:Ga₂O₃:ZnO=1:1:2 [molar ratio] or thelike can be used. Furthermore, a target having a composition ratio ofIn₂O₃:Ga₂O₃:ZnO=1:1:1 [molar ratio], a target having a composition ratioof In₂O₃:Ga₂O₃:ZnO=1:1:4 [molar ratio], or a target having a compositionratio of In₂O₃:Ga₂O₃:ZnO=1:0:2 [molar ratio] can also be used.

In this embodiment, the oxide semiconductor layer 144 having anamorphous structure is formed by a sputtering method with the use of anIn—Ga—Zn—O-based metal oxide target. The thickness ranges from 1 nm to50 nm, preferably from 2 nm to 20 nm, more preferably from 3 nm to 15nm.

The relative density of the metal oxide in the metal oxide target is 80%or more, preferably 95% or more, and more preferably 99.9% or more. Theuse of the metal oxide target having high relative density makes itpossible to form an oxide semiconductor layer having a dense structure.

The atmosphere in which the oxide semiconductor layer 144 is formed ispreferably a rare gas (typically, argon) atmosphere, an oxygenatmosphere, or a mixed atmosphere containing a rare gas (typically,argon) and oxygen. Specifically, it is preferable to use a high-puritygas atmosphere, for example, from which an impurity such as hydrogen,water, a hydroxyl group, or hydride is removed to 1 ppm or less(preferably, 10 ppb or less).

In forming the oxide semiconductor layer 144, for example, an object tobe processed is held in a treatment chamber that is maintained underreduced pressure, and the object to be processed is heated to atemperature higher than or equal to 100° C. and lower than 550° C.,preferably higher than or equal to 200° C. and lower than or equal to400° C. Alternatively, the temperature of an object to be processed informing the oxide semiconductor layer 144 may be room temperature (25°C.±10° C. (higher than or equal to 15° C. and lower than or equal to 35°C.)). Then, moisture in the treatment chamber is removed, a sputteringgas from which hydrogen, water, or the like has been removed isintroduced, and the above-described target is used; thus, the oxidesemiconductor layer 144 is formed. By forming the oxide semiconductorlayer 144 while heating the object to be processed, an impurity in theoxide semiconductor layer 144 can be reduced. Moreover, damage due tosputtering can be reduced. In order to remove the moisture in thetreatment chamber, it is preferable to use an entrapment vacuum pump.For example, a cryopump, an ion pump, a titanium sublimation pump, orthe like can be used. A turbo pump provided with a cold trap may beused. Since hydrogen, water, or the like can be removed from thetreatment chamber evacuated with a cryopump or the like, theconcentration of an impurity in the oxide semiconductor layer can bereduced.

For example, conditions for forming the oxide semiconductor layer 144can be set as follows: the distance between the object to be processedand the target is 170 mm, the pressure is 0.4 Pa, the direct current(DC) power is 0.5 kW, and the atmosphere is an oxygen (100% oxygen)atmosphere, an argon (100% argon) atmosphere, or a mixed atmosphere ofoxygen and argon. Note that a pulsed direct current (DC) power source ispreferably used because dust (powder or flake-like substances formed atthe time of the film formation) can be reduced and the film thicknesscan be made uniform. The thickness of the oxide semiconductor layer 144is set in the range of 1 nm to 50 nm, preferably 2 nm to 20 nm, morepreferably 3 nm to 15 nm. By employing a structure according to thedisclosed invention, a short-channel effect due to miniaturization canbe suppressed even in the case of using the oxide semiconductor layer144 having such a thickness. Note that the appropriate thickness of theoxide semiconductor layer differs depending on the oxide semiconductormaterial used, the intended use of the semiconductor device, or thelike; therefore, the thickness can be determined as appropriate inaccordance with the material, the intended use, or the like. Note that asurface where the oxide semiconductor layer 144 is formed issufficiently planarized in one embodiment of the disclosed invention.Therefore, even an oxide semiconductor layer having a small thicknesscan be favorably formed.

Note that before the oxide semiconductor layer 144 is formed by asputtering method, reverse sputtering in which plasma is generated withan argon gas introduced may be performed so that a material attached toa surface where the oxide semiconductor layer 144 is to be formed (e.g.,a surface of the insulating layer 143 b) is removed. Here, the reversesputtering is a method in which ions collide with a surface to beprocessed so that the surface is modified, in contrast to normalsputtering in which ions collide with a sputtering target. An example ofa method for making ions collide with a surface to be processed is amethod in which high-frequency voltage is applied to the surface side inan argon atmosphere so that plasma is generated near the object to beprocessed. Note that an atmosphere of nitrogen, helium, oxygen, or thelike may be used instead of an argon atmosphere.

After the oxide semiconductor layer 144 is formed, heat treatment (firstheat treatment) is preferably performed on the oxide semiconductor layer144. Through the first heat treatment, excess hydrogen (including wateror a hydroxyl group) in the oxide semiconductor layer 144 can beremoved, the structure of the oxide semiconductor layer 144 can beordered, and defect states in an energy gap can be reduced. For example,the temperature of the first heat treatment is set higher than or equalto 300° C. and lower than 550° C., or higher than or equal to 400° C.and lower than or equal to 500° C.

For example, after an object to be processed is introduced into anelectric furnace including a resistance heater or the like, the heattreatment can be performed at 450° C. for one hour in a nitrogenatmosphere. The oxide semiconductor layer is not exposed to the airduring the heat treatment so that entry of water or hydrogen can beprevented.

The heat treatment apparatus is not limited to the electric furnace andmay be an apparatus for heating an object to be processed by thermalradiation or thermal conduction from a medium such as a heated gas. Forexample, a rapid thermal annealing (RTA) apparatus such as a gas rapidthermal annealing (GRTA) apparatus or a lamp rapid thermal annealing(LRTA) apparatus can be used. The LRTA apparatus is an apparatus forheating an object to be processed by radiation of light (anelectromagnetic wave) emitted from a lamp such as a halogen lamp, ametal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressuresodium lamp, or a high pressure mercury lamp. The GRTA apparatus is anapparatus for performing heat treatment using a high-temperature gas. Asthe gas, an inert gas that does not react with an object to be processedby heat treatment, for example, nitrogen or a rare gas such as argon, isused.

For example, as the first heat treatment, GRTA treatment may beperformed as follows. The object to be processed is put in a heatedinert gas atmosphere, heated for several minutes, and taken out of theinert gas atmosphere. The GRTA treatment enables high-temperature heattreatment in a short time. Moreover, the GRTA treatment can be employedeven when the temperature exceeds the upper temperature limit of theobject to be processed. Note that the inert gas may be switched to a gasincluding oxygen during the treatment. This is because defect states inan energy gap caused by oxygen vacancies can be reduced by performingthe first heat treatment in an atmosphere including oxygen.

Note that as the inert gas atmosphere, an atmosphere that containsnitrogen or a rare gas (e.g., helium, neon, or argon) as its maincomponent and does not contain water, hydrogen, or the like ispreferably used. For example, the purity of nitrogen or a rare gas suchas helium, neon, or argon introduced into a heat treatment apparatus isset to 6N (99.9999%) or more, preferably 7N (99.99999%) or more (i.e.,the impurity concentration is 1 ppm or less, preferably 0.1 ppm orless).

In any case, a transistor with extremely excellent characteristics canbe obtained with the use of the oxide semiconductor layer which is ani-type (intrinsic) or substantially i-type oxide semiconductor layerobtained by reducing an impurity through the first heat treatment.

The above heat treatment (the first heat treatment) can also be referredto as dehydration treatment, dehydrogenation treatment, or the likebecause it has the effect of removing hydrogen, water, or the like. Thedehydration treatment or the dehydrogenation treatment can be performedafter the gate insulating layer 146 is formed, or after a gate electrodeis formed. Such dehydration treatment or dehydrogenation treatment maybe performed once or plural times.

After the oxide semiconductor layer 144 is formed, the oxidesemiconductor layer 144 may be processed into an island-shaped oxidesemiconductor layer. The oxide semiconductor layer 144 can be processedinto an island-shaped oxide semiconductor layer by etching, for example.The etching may be performed either before the heat treatment or afterthe heat treatment. Dry etching is preferably used in terms of elementminiaturization, but wet etching may be used. An etching gas or anetchant can be selected as appropriate depending on a material to beetched.

The gate insulating layer 146 can be formed by a CVD method, asputtering method, or the like. The gate insulating layer 146 ispreferably formed so as to contain silicon oxide, silicon nitride,silicon oxynitride, aluminum oxide, tantalum oxide, hafnium oxide,yttrium oxide, hafnium silicate (HfSi_(x)O_(y) (x>0, y>0)), hafniumsilicate to which nitrogen is added (HfSi_(x)O_(y)N_(z) (x>0, y>0,z>0)), hafnium aluminate to which nitrogen is added (HfAl_(x)O_(y)N_(z)(x>0, y>0, z>0)), or the like. The gate insulating layer 146 may have asingle-layer structure or a stacked-layer structure. There is noparticular limitation on the thickness of the gate insulating layer 146;the thickness is preferably small in order to ensure the operation ofthe transistor when the semiconductor device is miniaturized. Forexample, in the case of using silicon oxide, the thickness can be 1 nmto 100 nm, preferably 10 nm to 50 nm.

When the gate insulating layer is thin as described above, gate leakagedue to a tunnel effect or the like becomes a problem. In order to solvethe problem of gate leakage, the gate insulating layer 146 may be formedusing a high dielectric constant (high-k) material such as hafniumoxide, tantalum oxide, yttrium oxide, hafnium silicate (HfSi_(x)O_(y)(x>0, y>0)), hafnium silicate to which nitrogen is added(HfSi_(x)O_(y)N_(z) (x>0, y>0, z>0)), or hafnium aluminate to whichnitrogen is added (HfAl_(x)O_(y)N_(z) (x>0, y>0, z>0)). The use of ahigh-k material for the gate insulating layer 146 makes it possible toincrease the thickness in order to suppress gate leakage as well asensuring electrical properties. For example, the relative permittivityof hafnium oxide is approximately 15, which is much higher than that ofsilicon oxide which is 3 to 4. With such a material, a gate insulatinglayer where the equivalent oxide thickness is less than 15 nm,preferably 2 nm to 10 nm, can be easily formed. Note that astacked-layer structure of a film including a high-k material and a filmincluding any of silicon oxide, silicon nitride, silicon oxynitride,silicon nitride oxide, aluminum oxide, and the like may also beemployed.

After the gate insulating layer 146 is formed, second heat treatment ispreferably performed in an inert gas atmosphere or an oxygen atmosphere.The temperature of the heat treatment is set in the range of 200° C. to450° C., preferably 250° C. to 350° C. For example, the heat treatmentmay be performed at 250° C. for one hour in a nitrogen atmosphere. Bythe second heat treatment, variation in electrical characteristics ofthe transistor can be reduced. In the case where the gate insulatinglayer 146 contains oxygen, oxygen can be supplied to the oxidesemiconductor layer 144 and oxygen vacancies in the oxide semiconductorlayer 144 can be filled; thus, the oxide semiconductor layer 144 whichis i-type (intrinsic) or substantially i-type can also be formed.

Note that the second heat treatment is performed in this embodimentafter the gate insulating layer 146 is formed; there is no limitation onthe timing of the second heat treatment. For example, the second heattreatment may be performed after the gate electrode is formed.Alternatively, the first heat treatment and the second heat treatmentmay be performed in succession, or the first heat treatment may doubleas the second heat treatment, or the second heat treatment may double asthe first heat treatment.

By performing at least one of the first heat treatment and the secondheat treatment as described above, the oxide semiconductor layer 144 canbe purified so as not to contain impurities other than main componentsas little as possible.

Next, the gate electrode 148 a is formed over the gate insulating layer146 (see FIG. 2F). The gate electrode 148 a can be formed by forming aconductive layer over the gate insulating layer 146 and then selectivelyetching the conductive layer. The conductive layer to be the gateelectrode 148 a can be formed by a PVD method such as a sputteringmethod, or a CVD method such as a plasma CVD method. The details aresimilar to those in the case of the source electrode 142 a, the drainelectrode 142 b, or the like; thus, the description thereof can bereferred to. Note that although part of the gate electrode 148 aoverlaps with the source electrode 142 a and the drain electrode 142 bin the structure employed here, the disclosed invention is not limitedto this structure. It is possible to employ a structure in which an endportion of the gate electrode 148 a and an end portion of the sourceelectrode 142 a overlap with each other, and an end portion of the gateelectrode 148 a and an end portion of the drain electrode 142 b overlapwith each other.

Next, an insulating layer 150 and an insulating layer 152 are formed soas to cover the gate insulating layer 146, the gate electrode 148 a, andthe like (see FIG. 2G). The insulating layer 150 and the insulatinglayer 152 can be formed by a PVD method, a CVD method, or the like. Theinsulating layer 150 and the insulating layer 152 can be formed using amaterial including an inorganic insulating material such as siliconoxide, silicon oxynitride, silicon nitride, hafnium oxide, or aluminumoxide.

Note that the insulating layer 150 and the insulating layer 152 arepreferably formed using a low dielectric constant material or a lowdielectric constant structure (such as a porous structure). This isbecause when the insulating layer 150 and the insulating layer 152 havea low dielectric constant, capacitance generated between wirings,electrodes, or the like can be reduced and operation at higher speed canbe achieved.

Note that although a stacked-layer structure of the insulating layer 150and the insulating layer 152 is used in this embodiment, an embodimentof the disclosed invention is not limited to this example. Asingle-layer structure or a stacked-layer structure including three ormore layers can also be used. Alternatively, a structure in which theinsulating layers are not provided is also possible.

Note that the insulating layer 152 is desirably formed so as to have aflat surface. This is because when the insulating layer 152 has a flatsurface, an electrode, a wiring, or the like can be favorably formedover the insulating layer 152 even in the case where the semiconductordevice or the like is miniaturized. Note that the insulating layer 152can be planarized using a method such as chemical mechanical polishing(CMP).

Through the above steps, the transistor 162 including the oxidesemiconductor layer 144, which is purified, is completed (see FIG. 2G).

Note that a variety of wirings, electrodes, or the like may be formedafter the above steps. The wirings or the electrodes can be formed by amethod such as a so-called damascene method or dual damascene method.

As described above, in one embodiment of the disclosed invention, achannel formation region of the transistor 162 is provided over a highlyflat region with a root-mean-square (RMS) roughness of 1 nm or less.This makes it possible to prevent a problem such as a short-channeleffect even in a situation where the transistor 162 is miniaturized andto obtain the transistor 162 with favorable characteristics.

In addition, the oxide semiconductor layer 144 can have a uniformthickness by improving the planarity of the surface where components areformed, and the transistor 162 can have improved characteristics.Furthermore, a decrease in coverage which may be caused by a largedifference in height can be suppressed, and a disconnection or adefective connection of the oxide semiconductor layer 144 can beprevented.

Moreover, the path of electric current can be extended by making a smalldifference in height (e.g., 5 nm to 20 nm) between part of the uppersurface of the insulating layer 143 b and the upper surface of thesource electrode 142 a and between the part of the upper surface of theinsulating layer 143 b and the upper surface of the drain electrode 142b. This makes it possible to alleviate the concentration of an electricfield in the transistor 162 and to suppress a short-channel effect.

In the transistor 162 described in this embodiment, the oxidesemiconductor layer 144 is purified and thus contains hydrogen at aconcentration of 5×10¹⁹ atoms/cm³ or less, preferably 5×10¹⁸ atoms/cm³or less, more preferably 5×10¹⁷ atoms/cm³ or less. In addition, thedensity of carriers generated due to a donor such as hydrogen in theoxide semiconductor layer 144 is, for example, less than 1×10¹²/cm³,preferably less than 1.45×10¹⁰/cm³, which is sufficiently lower than thecarrier density of a general silicon wafer (approximately 1×10¹⁴/cm³).In addition, the off-state current of the transistor 162 is sufficientlysmall. For example, the off-state current (per unit channel width (1μm), here) of the transistor 162 at room temperature (25° C.) is 100 zA(1 zA (zeptoampere) is 1×10⁻²¹ A) or less, preferably 10 zA or less.When the above structure is employed, the off-state current of thetransistor can be 1×10⁻²⁴ A/μm to 1×10⁻³⁰ A/μm in theory.

In this manner, by using the oxide semiconductor layer 144 which ispurified and is intrinsic, it becomes easy to sufficiently reduce theoff-state current of the transistor. In addition, by using the oxidesemiconductor layer 144 which is purified and is intrinsic in thismanner, the S value of the transistor can be 65 mV/dec or less,preferably less than 63 mV/dec.

Next, FIGS. 3A to 3G will be described.

In a manner similar to the case of FIGS. 2A to 2G, first, a conductivelayer 241 a to be a source electrode later and a conductive layer 241 bto be a drain electrode later are formed over the substrate 240 having asurface where components are formed (see FIG. 3A). Then, an insulatinglayer 243 is formed so as to cover the conductive layer 241 a and theconductive layer 241 b (see FIG. 3B). The details are similar to thosein the case of FIGS. 2A and 2B.

Next, the insulating layer 243 b is formed by thinning the insulatinglayer 243 by chemical mechanical polishing (CMP) treatment (see FIG.3C). A difference from the case in FIG. 2C is to perform the CMPtreatment under such conditions that surfaces of the conductive layer241 a and the conductive layer 241 b become exposed. The otherconditions for the CMP treatment are similar to those in the case ofFIG. 2C. For details, the description about FIG. 2C can be referred to.

Next, the source electrode 242 a and the drain electrode 242 b areformed by thinning the conductive layer 241 a and the conductive layer241 b (see FIG. 3D). The conductive layer 241 a and the conductive layer241 b can be thinned by etching. For the etching, either dry etching orwet etching may be employed. Note that in order to ensure a sufficientplanarity of the surface of the insulating layer 243 b, the etchingneeds to be performed under such conditions that only conductive layerscan be selectively etched. An etching gas or an etchant can be selectedas appropriate depending on a material to be etched.

By the above thinning step, a surface having a small difference inheight (e.g., 5 nm to 20 nm) is formed by part of an upper surface ofthe insulating layer 243 b, an upper surface of the source electrode 242a, and an upper surface of the drain electrode 242 b. In addition, endportions of the source electrode 242 a and the drain electrode 242 b arenot covered with the insulating layer 243 b. Note that a side surface ofthe insulating layer 243 b may be inclined with respect to the bottomsurface.

Next, the oxide semiconductor layer 244 covering the above-describedsurface is formed in contact with part of the source electrode 242 a,the drain electrode 242 b, and the insulating layer 243 b; then, thegate insulating layer 246 is formed so as to cover the oxidesemiconductor layer 244 (see FIG. 3E). In addition, the gate electrode248 a is formed over the gate insulating layer 246 (see FIG. 3F). Then,an insulating layer 250 and an insulating layer 252 are formed so as tocover the gate insulating layer 246, the gate electrode 248 a, and thelike (see FIG. 3G). The details are similar to those in the case ofFIGS. 2E to 2G.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the structures, methods, and thelike described in the other embodiments.

Embodiment 2

In this embodiment, a structure and a manufacturing method of asemiconductor device according to another embodiment of the disclosedinvention will be described with reference to FIGS. 4A to 4C, FIGS. 5Ato 5D, and FIGS. 6A to 6C. Note that in some circuit diagrams mentionedbelow, “OS” is written beside a transistor in order to indicate that thetransistor includes an oxide semiconductor.

<Example of Structure of Semiconductor Device>

FIGS. 4A to 4C illustrate an example of a structure of a semiconductordevice. FIG. 4A is a cross-sectional view of the semiconductor device;FIG. 4B is a plan view of the semiconductor device; and FIG. 4Cillustrates a circuit configuration of the semiconductor device. Notethat a structure of the semiconductor device is mainly described in thisembodiment, and operation of the semiconductor device will be describedin detail in an embodiment below. Note that the semiconductor deviceillustrated in FIGS. 4A to 4C is just an example having predeterminedfunctions and does not represent all semiconductor devices according tothe disclosed invention. The semiconductor device according to thedisclosed invention can have another function by changing connectionrelationship of electrodes or the like as appropriate.

FIG. 4A corresponds to a cross-sectional view along line A1-A2 and lineB1-B2 in FIG. 4B. The semiconductor device illustrated in FIGS. 4A and4B includes the transistor 162 described in the above embodiment, atransistor 160 below the transistor 162, and a capacitor 164.

Here, a semiconductor material of the transistor 162 and a semiconductormaterial of the transistor 160 are preferably different materials. Forexample, the semiconductor material of the transistor 162 may be anoxide semiconductor, and the semiconductor material of the transistor160 may be a semiconductor material (such as silicon) other than anoxide semiconductor. A transistor including an oxide semiconductor canhold charge for a long time owing to its characteristics. On the otherhand, a transistor including a material other than an oxidesemiconductor can operate at high speed easily.

The transistor 160 in FIGS. 4A to 4C includes a channel formation region116 provided in a substrate 100 including a semiconductor material (suchas silicon), impurity regions 120 provided such that the channelformation region 116 is sandwiched therebetween, metal compound regions124 in contact with the impurity regions 120, a gate insulating layer108 provided over the channel formation region 116, and a gate electrode110 provided over the gate insulating layer 108. Note that a transistorwhose source electrode and drain electrode are not illustrated in adrawing may also be referred to as a transistor for the sake ofconvenience. Further, in such a case, in description of a connection ofa transistor, a source region and a source electrode may be collectivelyreferred to as a source electrode, and a drain region and a drainelectrode may be collectively referred to as a drain electrode. That is,in this specification, the term “source electrode” may include a sourceregion.

Further, an element isolation insulating layer 106 is formed over thesubstrate 100 so as to surround the transistor 160, and an insulatinglayer 130 is formed to cover the transistor 160. Note that in order torealize higher integration, the transistor 160 preferably has astructure without a sidewall insulating layer as illustrated in FIGS. 4Aand 4B. On the other hand, in the case where characteristics of thetransistor 160 have priority, a sidewall insulating layer may beprovided on a side surface of the gate electrode 110, and the impurityregions 120 may include a region having a different impurityconcentration.

The structure of the transistor 162 in FIGS. 4A to 4C is similar to thestructure of the transistor 162 in the above embodiment. Note that inthis embodiment, the source electrode 142 a (which may be the drainelectrode) of the transistor 162 is connected to the gate electrode 110of the transistor 160. It is needless to say that the transistor 162 canbe replaced with the transistor 262.

The capacitor 164 in FIGS. 4A to 4C includes the source electrode 142 a(which may be the drain electrode), the oxide semiconductor layer 144,the gate insulating layer 146, and an electrode 148 b. In other words,the source electrode 142 a functions as one electrode of the capacitor164, and the electrode 148 b functions as the other electrode of thecapacitor 164. Note that the electrode 148 b is formed in a processsimilar to that of the gate electrode 148 a of the transistor 162.

Note that in the capacitor 164 of FIGS. 4A to 4C, the oxidesemiconductor layer 144 and the gate insulating layer 146 are stacked,whereby insulation between the source electrode 142 a and the electrode148 b can be sufficiently secured. It is needless to say that thecapacitor 164 without including the oxide semiconductor layer 144 may beemployed in order to secure sufficient capacitance. In addition, in thecase where no capacitor is needed, a structure in which the capacitor164 is not provided is also possible.

In this embodiment, the transistor 162 and the capacitor 164 areprovided so as to overlap with the transistor 160. By employing such aplanar layout, higher integration can be realized. For example, giventhat the minimum feature size is F, the area occupied by thesemiconductor device can be 15 F² to 25 F².

Note that the structure of a semiconductor device according to thedisclosed invention is not limited to that illustrated in FIGS. 4A to4C. Since the technical idea of the disclosed invention is to form astacked-layer structure with an oxide semiconductor and a material otherthan an oxide semiconductor, the details of connection relationship ofelectrodes or the like can be changed as appropriate.

<Example of Method for Manufacturing Semiconductor Device>

Next, an example of a method for manufacturing the semiconductor devicewill be described with reference to FIGS. 5A to 5D and FIGS. 6A to 6C.Note that a method for manufacturing the transistor 162 is similar tothat in the above embodiment; thus, a method for manufacturing thetransistor 160 will be mainly described here.

First, the substrate 100 including a semiconductor material is prepared(see FIG. 5A). A single crystal semiconductor substrate or apolycrystalline semiconductor substrate of silicon, silicon carbide, orthe like, a compound semiconductor substrate of silicon germanium or thelike, an SOI substrate, or the like can be used as the substrate 100including a semiconductor material. Here, an example of the case where asingle crystal silicon substrate is used as the substrate 100 includinga semiconductor material is described. Note that the term “SOIsubstrate” generally means a substrate where a silicon semiconductorlayer is provided over an insulating surface. In this specification, theterm “SOI substrate” also means a substrate where a semiconductor layerincluding a material other than silicon is provided over an insulatingsurface. That is, a semiconductor layer included in the “SOI substrate”is not limited to a silicon semiconductor layer. Moreover, the SOIsubstrate can be a substrate having a structure where a semiconductorlayer is provided over an insulating substrate such as a glass substratewith an insulating layer interposed therebetween.

It is preferable that a single crystal semiconductor substrate ofsilicon or the like be particularly used as the substrate 100 includinga semiconductor material because the speed of reading operation of thesemiconductor device can be increased.

Note that an impurity element may be added to a region which laterfunctions as the channel formation region 116 of the transistor 160, inorder to control the threshold voltage of the transistor. Here, animpurity element imparting conductivity is added so that the thresholdvoltage of the transistor 160 becomes positive. When the semiconductormaterial is silicon, the impurity imparting conductivity may be boron,aluminum, gallium, or the like. Note that it is preferable to performheat treatment after adding an impurity element, in order to activatethe impurity element or reduce defects which may be generated duringaddition of the impurity element.

Next, a protective layer 102 serving as a mask for forming an elementisolation insulating layer is formed over the substrate 100 (see FIG.5A). As the protective layer 102, an insulating layer formed using amaterial such as silicon oxide, silicon nitride, silicon oxynitride, orthe like can be used, for example.

Next, part of the substrate 100 in a region not covered with theprotective layer 102 (i.e., in an exposed region) is removed by etchingusing the protective layer 102 as a mask. Thus, a semiconductor region104 isolated from other semiconductor regions is formed (see FIG. 5B).As the etching, dry etching is preferably performed, but wet etching maybe performed. An etching gas or an etchant can be selected asappropriate depending on a material to be etched.

Then, an insulating layer is formed so as to cover the semiconductorregion 104, and the insulating layer in a region overlapping with thesemiconductor region 104 is selectively removed; thus, the elementisolation insulating layer 106 is formed (see FIG. 5C). The insulatinglayer is formed using silicon oxide, silicon nitride, siliconoxynitride, or the like. As a method for removing the insulating layer,any of etching treatment, polishing treatment such as chemicalmechanical polishing (CMP) treatment, and the like can be employed. Notethat the protective layer 102 is removed after the formation of thesemiconductor region 104 or after the formation of the element isolationinsulating layer 106.

Next, an insulating layer is formed over a surface of the semiconductorregion 104, and a layer including a conductive material is formed overthe insulating layer.

The insulating layer is processed into a gate insulating layer later andcan be formed by heat treatment (thermal oxidation treatment, thermalnitridation treatment, or the like) of the surface of the semiconductorregion 104, for example. Instead of heat treatment, high-density plasmatreatment may be employed. The high-density plasma treatment can beperformed using, for example, a mixed gas of any of a rare gas such ashelium (He), argon (Ar), krypton (Kr), or xenon (Xe), oxygen, nitrogenoxide, ammonia, nitrogen, hydrogen, and the like. It is needless to saythat the insulating layer may be formed by a CVD method, a sputteringmethod, or the like. The insulating layer preferably has a single-layerstructure or a stacked-layer structure with a film including siliconoxide, silicon oxynitride, silicon nitride, hafnium oxide, aluminumoxide, tantalum oxide, yttrium oxide, hafnium silicate (HfSi_(x)O_(y)(x>0, y>0)), hafnium silicate to which nitrogen is added(HfSi_(x)O_(y)N, (x>0, y>0, z>0)), hafnium aluminate to which nitrogenis added (HfAl_(x)O_(y)N_(z) (x>0, y>0, z>0)), or the like. Theinsulating layer can have a thickness of 1 nm to 100 nm, preferably, 10nm to 50 nm, for example.

The layer including a conductive material can be formed using a metalmaterial such as aluminum, copper, titanium, tantalum, or tungsten. Thelayer including a conductive material may be formed using asemiconductor material such as polycrystalline silicon. There is noparticular limitation on the method for forming the layer including aconductive material, and a variety of film formation methods such as anevaporation method, a CVD method, a sputtering method, or a spin coatingmethod can be employed. Note that this embodiment shows an example ofthe case where the layer including a conductive material is formed usinga metal material.

After that, the insulating layer and the layer including a conductivematerial are selectively etched; thus, the gate insulating layer 108 andthe gate electrode 110 are formed (see FIG. 5C).

Next, phosphorus (P), arsenic (As), or the like is added to thesemiconductor region 104, whereby the channel formation region 116 andthe impurity regions 120 are formed (see FIG. 5D). Note that phosphorusor arsenic is added here in order to form an n-type transistor; animpurity element such as boron (B) or aluminum (Al) may be added in thecase of forming a p-type transistor. Here, the concentration of theimpurity added can be set as appropriate; the concentration ispreferably set high when a semiconductor element is highly miniaturized.

Note that a sidewall insulating layer may be formed around the gateelectrode 110, and impurity regions to which the impurity element isadded at a different concentration may be formed.

Next, a metal layer 122 is formed so as to cover the gate electrode 110,the impurity regions 120, and the like (see FIG. 6A). The metal layer122 can be formed by a variety of film formation methods such as avacuum evaporation method, a sputtering method, and a spin coatingmethod. The metal layer 122 is preferably formed using a metal materialwhich forms a low-resistance metal compound by reacting with thesemiconductor material included in the semiconductor region 104.Examples of such metal materials are titanium, tantalum, tungsten,nickel, cobalt, platinum, and the like.

Next, heat treatment is performed so that the metal layer 122 reactswith the semiconductor material. Thus, the metal compound regions 124that are in contact with the impurity regions 120 are formed (see FIG.6A). Note that when the gate electrode 110 is formed usingpolycrystalline silicon or the like, a metal compound region is alsoformed in a portion of the gate electrode 110 which is in contact withthe metal layer 122.

As the heat treatment, irradiation with a flash lamp can be employed,for example. Although it is needless to say that another heat treatmentmethod may be used, a method by which heat treatment can be achieved inan extremely short time is preferably used in order to improve thecontrollability of chemical reaction for formation of the metalcompound. Note that the metal compound regions are formed by reaction ofthe metal material and the semiconductor material and have sufficientlyhigh conductivity. The formation of the metal compound regions canproperly reduce the electric resistance and improve elementcharacteristics. Note that the metal layer 122 is removed after themetal compound regions 124 are formed.

Next, the insulating layer 130 is formed so as to cover the componentsformed in the above steps (see FIG. 6B). The insulating layer 130 can beformed using an inorganic insulating material such as silicon oxide,silicon oxynitride, silicon nitride, or aluminum oxide. It isparticularly preferable to use a low dielectric constant (low-k)material for the insulating layer 130 because capacitance due to overlapof electrodes or wirings can be sufficiently reduced. Note that a porousinsulating layer with such a material may be employed as the insulatinglayer 130. The porous insulating layer has a lower dielectric constantthan an insulating layer with high density and thus makes it possible tofurther reduce capacitance due to electrodes or wirings. Alternatively,the insulating layer 130 can be formed using an organic insulatingmaterial such as a polyimide or an acrylic resin. Note that although asingle-layer structure of the insulating layer 130 is used in thisembodiment, an embodiment of the disclosed invention is not limited tothis example. A stacked-layer structure with two or more layers may beemployed.

Through the above steps, the transistor 160 is formed with the use ofthe substrate 100 including a semiconductor material (see FIG. 6B). Afeature of the transistor 160 is that it can operate at high speed. Withthe use of that transistor as a transistor for reading, data can be readat high speed.

After that, as treatment performed before the transistor 162 and thecapacitor 164 are formed, CMP treatment of the insulating layer 130 isperformed so that an upper surface of the gate electrode 110 is exposed(see FIG. 6C). As treatment for exposing the upper surface of the gateelectrode 110, etching treatment or the like can also be employedinstead of CMP treatment; in order to improve characteristics of thetransistor 162, a surface of the insulating layer 130 is preferably madeas flat as possible. For example, the insulating layer 130 is planarizedsuch that its surface has a root-mean-square (RMS) roughness of 1 nm orless. This makes it possible to improve characteristics of asemiconductor device formed over the insulating layer 130.

Note that before or after each of the above steps, a step of forming anelectrode, a wiring, a semiconductor layer, an insulating layer, or thelike may be further performed. For example, when the wiring has amulti-layer structure of a stacked-layer structure including insulatinglayers and conductive layers, a highly integrated semiconductor devicecan be realized.

After that, the transistor 162 and the capacitor 164 are formed; thus,the semiconductor device is completed.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the structures, methods, and thelike described in the other embodiments.

Embodiment 3

In this embodiment, an example of application of a semiconductor deviceaccording to one embodiment of the disclosed invention will be describedwith reference to FIGS. 7A-1, 7A-2, and 7B. Here, an example of a memorydevice is described. Note that in some circuit diagrams mentioned below,“OS” is written beside a transistor in order to indicate that thetransistor includes an oxide semiconductor.

In a semiconductor device which can be used as a memory device, which isillustrated in FIG. 7A-1, a first wiring (1st Line) is electricallyconnected to a source electrode of a transistor 1000. A second wiring(2nd Line) is electrically connected to a drain electrode of thetransistor 1000. A third wiring (3rd Line) is electrically connected toone of a source electrode and a drain electrode of a transistor 1010. Afourth wiring (4th Line) is electrically connected to a gate electrodeof the transistor 1010. Furthermore, a gate electrode of the transistor1000 and the other of the source electrode and the drain electrode ofthe transistor 1010 are electrically connected to one electrode of acapacitor 1020. A fifth wiring (5th Line) is electrically connected tothe other electrode of the capacitor 1020.

Here, a transistor including an oxide semiconductor is used as thetransistor 1010. Any of the transistors described in the aboveembodiments can be used as the transistor including an oxidesemiconductor. A transistor including an oxide semiconductor has acharacteristic of a significantly small off-state current. For thatreason, a potential of the gate electrode of the transistor 1000 can beheld for an extremely long period even if the transistor 1010 is turnedoff. Furthermore, with the use of any of the transistors described inthe above embodiments, the short-channel effect of the transistor 1010can be suppressed, and miniaturization can be achieved. By providing thecapacitor 1020, holding of charge applied to the gate electrode of thetransistor 1000 and reading of data held can be performed more easily.Here, the capacitor described in the above embodiment can be used as thecapacitor 1020, for example.

A transistor including a semiconductor material other than an oxidesemiconductor is used as the transistor 1000. As the semiconductormaterial other than an oxide semiconductor, for example, silicon,germanium, silicon germanium, silicon carbide, gallium arsenide, or thelike can be used, and a single crystal semiconductor is preferably used.Alternatively, an organic semiconductor material or the like may beused. A transistor including such a semiconductor material can operateat high speed. Here, any of the transistors described in the aboveembodiments can be used as the transistor including a semiconductormaterial other than an oxide semiconductor, for example.

Alternatively, a structure in which the capacitor 1020 is not providedis also possible as illustrated in FIG. 7B.

The semiconductor device in FIG. 7A-1 utilizes a characteristic in whichthe potential of the gate electrode of the transistor 1000 can be held,and can thus write, hold, and read data as follows.

First of all, writing and holding of data will be described. First, thepotential of the fourth wiring is set to a potential at which thetransistor 1010 is turned on, so that the transistor 1010 is turned on.Accordingly, the potential of the third wiring is supplied to the gateelectrode of the transistor 1000 and to the capacitor 1020. That is,predetermined charge is supplied to the gate electrode of the transistor1000 (writing). Here, one of two kinds of charges providing differentpotentials (hereinafter, a charge providing a low potential is referredto as charge Q_(L) and a charge providing a high potential is referredto as charge Q_(H)) is applied. Note that three or more kinds of chargesproviding different potentials may be applied in order to improvestorage capacity. After that, the potential of the fourth wiring is setto a potential at which the transistor 1010 is turned off, so that thetransistor 1010 is turned off. Thus, the charge supplied to the gateelectrode of the transistor 1000 is held (holding).

Since the off-state current of the transistor 1010 is significantlysmall, the charge of the gate electrode of the transistor 1000 is heldfor a long time.

Next, reading of data will be described. By supplying an appropriatepotential (reading potential) to the fifth wiring while a predeterminedpotential (a constant potential) is supplied to the first wiring, thepotential of the second wiring varies depending on the amount of chargeheld at the gate electrode of the transistor 1000. This is generallybecause, when the transistor 1000 is an n-channel transistor, anapparent threshold voltage V_(th) _(—) _(H) in the case where Q_(H) issupplied to the gate electrode of the transistor 1000 is lower than anapparent threshold voltage V_(th) _(—) _(L) in the case where Q_(L) issupplied to the gate electrode of the transistor 1000. Here, an apparentthreshold voltage refers to the potential of the fifth wiring, which isneeded to turn on the transistor 1000. Thus, the potential of the fifthwiring is set to a potential V₀ intermediate between V_(th) _(—) _(x)and V_(th) _(—) _(L), whereby charge supplied to the gate electrode ofthe transistor 1000 can be determined. For example, in the case whereQ_(H) is supplied in writing, when the potential of the fifth wiring isV₀ (>V_(th) _(—) _(H)), the transistor 1000 is turned on. In the casewhere Q_(L) is supplied in writing, even when the potential of the fifthwiring is V_(o) (<V_(th) _(—) _(L)), the transistor 1000 remains off.Therefore, the data held can be read by measuring the potential of thesecond wiring.

Note that in the case where memory cells are arrayed to be used, it isnecessary that only data of a desired memory cell can be read. In orderto read data of a predetermined memory cell and not to read data of theother memory cells, in the case where the transistors 1000 are connectedin parallel among the memory cells, fifth wirings in memory cells thatare not a target for reading are supplied with a potential at which thetransistors 1000 are turned off regardless of the state of the gateelectrodes, that is, a potential lower than V_(th) _(—) _(H). On theother hand, in the case where the transistors 1000 are connected inseries among the memory cells, fifth wirings in memory cells that arenot a target for reading are supplied with a potential at which thetransistors 1000 are turned on regardless of the state of the gateelectrodes, that is, a potential higher than V_(th) _(—) _(L).

Next, rewriting of data will be described. Rewriting of data isperformed in a manner similar to that of the writing and holding ofdata. That is, the potential of the fourth wiring is set to a potentialat which the transistor 1010 is turned on, so that the transistor 1010is turned on. Accordingly, the potential of the third wiring (apotential for new data) is supplied to the gate electrode of thetransistor 1000 and to the capacitor 1020. After that, the potential ofthe fourth wiring is set to a potential at which the transistor 1010 isturned off, so that the transistor 1010 is turned off. Accordingly,charge for new data is supplied to the gate electrode of the transistor1000.

In the semiconductor device according to the disclosed invention, datacan be directly rewritten by another writing of data as described above.Therefore, extraction of charge from a floating gate with the use of ahigh voltage which is necessary for a flash memory or the like is notneeded, and thus a decrease in operation speed due to erasing operationcan be suppressed. In other words, high-speed operation of thesemiconductor device can be realized.

Note that the source electrode or the drain electrode of the transistor1010 is electrically connected to the gate electrode of the transistor1000 and therefore has a function similar to that of a floating gate ofa floating gate transistor used for a nonvolatile memory element.Therefore, in drawings, a portion where the source electrode or thedrain electrode of the transistor 1010 is electrically connected to thegate electrode of the transistor 1000 is called a floating gate portionFG in some cases. When the transistor 1010 is turned off, the floatinggate portion FG can be regarded as being embedded in an insulator andthus charge is held in the floating gate portion FG The off-statecurrent of the transistor 1010 including an oxide semiconductor issmaller than or equal to 1/100000 of the off-state current of atransistor including a silicon semiconductor or the like; thus, loss ofthe charge accumulated in the floating gate portion FG due to leakage ofthe transistor 1010 is negligible. That is, with the transistor 1010including an oxide semiconductor, a nonvolatile memory device which canhold data without being supplied with power can be realized.

For example, when the off-state current of the transistor 1010 at roomtemperature is 10 zA (1 zA (zeptoampere) is 1×10⁻²¹ A) or less and thecapacitance of the capacitor 1020 is approximately 10 fF, data can beheld for 10⁴ seconds or longer. It is needless to say that the holdingtime depends on transistor characteristics and capacitance.

Further, in that case, the problem of deterioration of a gate insulatingfilm (tunnel insulating film), which is a problem of a conventionalfloating gate transistor, does not exist. That is, the problem ofdeterioration of a gate insulating film due to injection of electronsinto a floating gate, which is a conventional problem, can be solved.This means that there is no limit on the number of write cycles inprinciple. Furthermore, a high voltage needed for writing or erasing ina conventional floating gate transistor is not necessary.

Components such as transistors in the semiconductor device in FIG. 7A-1can be regarded as including resistors and capacitors as illustrated inFIG. 7A-2. That is, in FIG. 7A-2, the transistor 1000 and the capacitor1020 are each regarded as including a resistor and a capacitor. R1 andC1 denote the resistance and the capacitance of the capacitor 1020,respectively. The resistance R1 corresponds to the resistance of theinsulating layer included in the capacitor 1020. R2 and C2 denote theresistance and the capacitance of the transistor 1000, respectively. Theresistance R2 corresponds to the resistance of the gate insulating layerat the time when the transistor 1000 is turned on. The capacitance C2corresponds to a so-called gate capacitance (capacitance formed betweenthe gate electrode and the source or drain electrode, and capacitanceformed between the gate electrode and the channel formation region).

A charge holding period (also referred to as a data holding period) isdetermined mainly by the off-state current of the transistor 1010 underthe conditions where the gate leakage current of the transistor 1010 issufficiently small and R1 and R2 satisfy R1≧ROS (R1 is greater than orequal to ROS) and R2 ROS (R2 is greater than or equal to ROS), where ROSis the resistance (also referred to as effective resistance) between thesource electrode and the drain electrode in a state where the transistor1010 is turned off.

On the other hand, in the case where the above conditions are notsatisfied, it is difficult to secure a sufficient holding period even ifthe off-state current of the transistor 1010 is sufficiently small. Thisis because leakage current other than the off-state current of thetransistor 1010 (e.g., leakage current generated between the sourceelectrode and the gate electrode) is large. Accordingly, it can be saidthat the semiconductor device disclosed in this embodiment preferablysatisfies the above relationships.

Meanwhile, it is desirable that C1 and C2 satisfy C1 C2 (C1 is greaterthan or equal to C2). This is because if C1 is larger than or equal toC2, when the potential of the floating gate portion FG is controlled bythe fifth wiring, the potential of the fifth wiring can be efficientlysupplied to the floating gate portion FG and the difference betweenpotentials supplied to the fifth wiring (e.g., a reading potential and anon-reading potential) can be kept small.

When the above relationships are satisfied, a more favorablesemiconductor device can be realized. Note that R1 and R2 depend on thegate insulating layer of the transistor 1000 and the insulating layer ofthe capacitor 1020. The same dependence applies to C1 and C2. Therefore,the material, the thickness, and the like of the gate insulating layerare preferably set as appropriate to satisfy the above relationships.

In the semiconductor device described in this embodiment, the floatinggate portion FG has a function similar to a floating gate of a floatinggate transistor of a flash memory or the like, but the floating gateportion FG of this embodiment has a feature which is essentiallydifferent from that of the floating gate of the flash memory or thelike. In the case of a flash memory, since a high potential is appliedto a control gate, it is necessary to keep a proper distance betweencells in order to prevent the potential of the control gate fromaffecting a floating gate of an adjacent cell. This is one factorinhibiting higher integration of the semiconductor device. The factor isattributed to a basic principle of a flash memory, in which a tunnelingcurrent is generated by applying a high electric field.

Further, because of the above principle of a flash memory, deteriorationof an insulating film proceeds and thus another problem that is thelimit on the number of times of rewriting (approximately 10⁴ to 10⁵times) arises.

The semiconductor device according to the disclosed invention isoperated by switching of a transistor including an oxide semiconductorand does not use the above-described principle of charge injection by atunneling current. That is, a high electric field for charge injectionis not necessary, unlike a flash memory. Accordingly, it is notnecessary to consider an influence of a high electric field from acontrol gate on an adjacent cell, and this facilitates higherintegration.

Further, charge injection by a tunneling current is not employed, whichmeans that there are no causes for deterioration of a memory cell. Inother words, the semiconductor device according to the disclosedinvention has higher durability and reliability than a flash memory.

In addition, the semiconductor device according to the disclosedinvention is advantageous over a flash memory also in that a largestep-up circuit or the like is not necessary because a high electricfield is not necessary.

In the case where the relative permittivity ∈r1 of the insulating layerincluded in the capacitor 1020 is different from the relativepermittivity ∈r2 of the insulating layer included in the transistor1000, it is easy to satisfy C1≧C2 (C1 is greater than or equal to C2)while satisfying 2·S2≧S1 (2·S2 is greater than or equal to S1),desirably S2 S1 (S2 is greater than or equal to S1), where S1 is thearea of the insulating layer included in the capacitor 1020 and S2 isthe area of the insulating layer forming a gate capacitor of thetransistor 1000. In other words, C1 can easily be made greater than orequal to C2 while the area of the insulating layer included in thecapacitor 1020 is small. Specifically, for example, a film including ahigh-k material such as hafnium oxide or a stack of a film including ahigh-k material such as hafnium oxide and a film including an oxidesemiconductor is used for the insulating layer included in the capacitor1020 so that ∈r1 can be set to 10 or more, preferably 15 or more, andsilicon oxide is used for the insulating layer forming the gatecapacitor so that 3≦∈r2≧4 (∈r2 is greater than or equal to 3 and lessthan or equal to 4).

A combination of such structures enables further higher integration ofthe semiconductor device according to the disclosed invention.

Note that an n-type transistor (n-channel transistor) in which electronsare majority carriers is used in the above description; it is needlessto say that a p-type transistor (p-channel transistor) in which holesare majority carriers can be used instead of the n-type transistor.

As described above, a semiconductor device according to an embodiment ofthe disclosed invention has a nonvolatile memory cell including awriting transistor where a leakage current (off-state current) between asource and a drain in an off state is small, a reading transistorincluding a semiconductor material different from that of the writingtransistor, and a capacitor.

With a normal silicon semiconductor, it is difficult to decrease theleakage current (the off-state current) to 100 zA (1×10⁻¹⁹ A) or less atambient temperature (e.g., 25° C.), whereas this value can be achievedwith a transistor including an oxide semiconductor which is processedunder appropriate conditions. Therefore, a transistor including an oxidesemiconductor is preferably used as the writing transistor.

In addition, a transistor including an oxide semiconductor has a smallsubthreshold swing (S value), so that the switching speed can besufficiently increased even if the mobility is relatively low.Therefore, by using the transistor as the writing transistor, rising ofa writing pulse supplied to the floating gate portion FG can be madevery sharp. Further, because of such a small off-state current, theamount of charge required to be held in the floating gate portion FG canbe reduced. That is, by using a transistor including an oxidesemiconductor as the writing transistor, rewriting of data can beperformed at high speed.

There is no limitation on the off-state current of the readingtransistor; it is desirable to use a transistor which operates at highspeed in order to increase the reading speed. For example, a transistorwith a switching speed of 1 nanosecond or less is preferably used as thereading transistor.

In this manner, when a transistor including an oxide semiconductor isused as a writing transistor and a transistor including a semiconductormaterial other than an oxide semiconductor is used as a readingtransistor, a semiconductor device capable of holding data for a longtime and reading data at high speed, which can be used as a memorydevice, can be obtained.

Furthermore, with the use of any of the transistors described in theabove embodiments as a writing transistor, the short-channel effect ofthe writing transistor can be suppressed, and miniaturization can beachieved. Accordingly, a semiconductor device which can be used as amemory device can have higher integration.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the structures, methods, and thelike described in the other embodiments.

Embodiment 4

In this embodiment, an example of application of a semiconductor deviceaccording to one embodiment of the disclosed invention will be describedwith reference to FIGS. 8A and 8B and FIGS. 9A to 9C. Here, an exampleof a memory device is described. Note that in some circuit diagramsmentioned below, “OS” is written beside a transistor in order toindicate that the transistor includes an oxide semiconductor.

FIGS. 8A and 8B are circuit diagrams of semiconductor devices, which canbe used as memory devices, each including a plurality of semiconductordevices (hereinafter also referred to as memory cells 1050) illustratedin FIG. 7A-1. FIG. 8A is a circuit diagram of a so-called NANDsemiconductor device in which the memory cells 1050 are connected inseries, and FIG. 8B is a circuit diagram of a so-called NORsemiconductor device in which the memory cells 1050 are connected inparallel.

The semiconductor device in FIG. 8A includes a source line SL, a bitline BL, a first signal line S1, m second signal lines S2, m word linesWL, and m memory cells 1050. In FIG. 8A, one source line SL and one bitline BL are provided in the semiconductor device; however, an embodimentof the disclosed invention is not limited to this structure. A pluralityof source lines SL and a plurality of bit lines BL may be provided.

In each of the memory cells 1050, the gate electrode of the transistor1000, one of the source electrode and the drain electrode of thetransistor 1010, and one electrode of the capacitor 1020 areelectrically connected to one another. The first signal line 51 and theother of the source electrode and the drain electrode of the transistor1010 are electrically connected to each other, and the second signalline S2 and the gate electrode of the transistor 1010 are electricallyconnected to each other. The word line WL and the other electrode of thecapacitor 1020 are electrically connected to each other.

Further, the source electrode of the transistor 1000 included in thememory cell 1050 is electrically connected to the drain electrode of thetransistor 1000 in the adjacent memory cell 1050. The drain electrode ofthe transistor 1000 included in the memory cell 1050 is electricallyconnected to the source electrode of the transistor 1000 in the adjacentmemory cell 1050. Note that the drain electrode of the transistor 1000included in the memory cell 1050 at one end of the plurality of memorycells connected in series is electrically connected to the bit line BL.The source electrode of the transistor 1000 included in the memory cell1050 at the other end of the plurality of memory cells connected inseries is electrically connected to the source line SL.

In the semiconductor device in FIG. 8A, writing operation and readingoperation are performed for each row. The writing operation is performedas follows. A potential at which the transistor 1010 is turned on issupplied to the second signal line S2 of a row where writing is to beperformed, so that the transistor 1010 of the row where writing is to beperformed is turned on. Accordingly, a potential of the first signalline S1 is supplied to the gate electrode of the transistor 1000 of thespecified row, so that predetermined charge is applied to the gateelectrode. Thus, data can be written to the memory cell of the specifiedrow.

Further, the reading operation is performed as follows. First, apotential at which the transistor 1000 is turned on regardless of chargeof the gate electrode thereof is supplied to the word lines WL of therows other than the row where reading is to be performed, so that thetransistors 1000 of the rows other than the row where reading is to beperformed are turned on. Then, a potential (reading potential) at whichan on state or an off state of the transistor 1000 is determineddepending on charge of the gate electrode of the transistor 1000 issupplied to the word line WL of the row where reading is to beperformed. After that, a constant potential is supplied to the sourceline SL so that a reading circuit (not illustrated) connected to the bitline BL is operated. Here, the plurality of transistors 1000 between thesource line SL and the bit line BL are turned on except the transistor1000 of the row where reading is to be performed; therefore, conductancebetween the source line SL and the bit line BL is determined by thestate of the transistor 1000 (whether being turned on or off) of the rowwhere reading is to be performed. Since the conductance of thetransistor varies depending on the electric charge in the gate electrodeof the transistor 1000 of the row where reading is to be performed, apotential of the bit line BL also varies accordingly. By reading thepotential of the bit line BL with the reading circuit, data can be readfrom the memory cell of the specified row.

The semiconductor device in FIG. 8B includes n source lines SL, n bitlines BL, n first signal lines S1, m second signal lines S2, m wordlines WL, and n×m memory cells 1050. A gate electrode of the transistor1000, one of the source electrode and the drain electrode of thetransistor 1010, and one electrode of the capacitor 1020 areelectrically connected to one another. The source line SL and the sourceelectrode of the transistor 1000 are electrically connected to eachother. The bit line BL and the drain electrode of the transistor 1000are electrically connected to each other. The first signal line 51 andthe other of the source electrode and the drain electrode of thetransistor 1010 are electrically connected to each other, and the secondsignal line S2 and the gate electrode of the transistor 1010 areelectrically connected to each other. The word line WL and the otherelectrode of the capacitor 1020 are electrically connected to eachother.

In the semiconductor device in FIG. 8B, writing operation and readingoperation are performed for each row. The writing operation is performedin a manner similar to that of the semiconductor device in FIG. 8A. Thereading operation is performed as follows. First, a potential at whichthe transistor 1000 is turned off regardless of charge of the gateelectrode thereof is supplied to the word lines WL of the rows otherthan the row where reading is to be performed, so that the transistors1000 of the rows other than the row where reading is to be performed areturned off. Then, a potential (reading potential) at which an on stateor an off state of the transistor 1000 is determined depending on chargeof the gate electrode of the transistor 1000 is supplied to the wordline WL of the row where reading is to be performed. After that, aconstant potential is supplied to the source line SL so that a readingcircuit (not illustrated) connected to the bit line BL is operated.Here, conductance between the source line SL and the bit line BL isdetermined by the state of the transistor 1000 (whether being turned onor off) of the row where reading is to be performed. That is, apotential of the bit line BL depends on charge of the gate electrode ofthe transistor 1000 of the row where reading is to be performed. Byreading the potential of the bit line BL with the reading circuit, datacan be read from the memory cell of the specified row.

Although the amount of data which can be stored in each of the memorycells 1050 is one bit in the above description, the structure of thesemiconductor device of this embodiment is not limited to this example.The amount of data which is held in each of the memory cells 1050 may beincreased by preparing three or more kinds of potentials to be suppliedto the gate electrode of the transistor 1000. For example, in the casewhere four kinds of potentials are supplied to the gate electrode of thetransistor 1000, data of two bits can be held in each of the memorycells.

Next, an example of a reading circuit which can be used for thesemiconductor devices illustrated in FIGS. 8A and 8B and the like willbe described with reference to FIGS. 9A to 9C.

FIG. 9A illustrates an outline of the reading circuit. The readingcircuit includes a transistor and a sense amplifier circuit.

At the time of reading data, a terminal A is connected to a bit line BLto which a memory cell from which data is to be read is connected.Further, a bias potential Vbias is applied to a gate electrode of thetransistor so that a potential of the terminal A is controlled.

The resistance of the memory cell 1050 varies depending on data stored.Specifically, when the transistor 1000 of the memory cell 1050 selectedis turned on, the memory cell 1050 has a low resistance, whereas whenthe transistor 1000 of the memory cell 1050 selected is turned off, thememory cell 1050 has a high resistance.

When the memory cell has a high resistance, the potential of theterminal A is higher than a reference potential Vref and the senseamplifier circuit outputs a potential corresponding to the potential ofthe terminal A. On the other hand, when the memory cell has a lowresistance, the potential of the terminal A is lower than the referencepotential Vref and the sense amplifier circuit outputs a potentialcorresponding to the potential of the terminal A.

In this manner, by using the reading circuit, data can be read from thememory cell. Note that the reading circuit of this embodiment is oneexample. Another circuit may be used. The reading circuit may furtherinclude a precharge circuit. Instead of the reference potential Vref, areference bit line may be connected to the sense amplifier circuit.

FIG. 9B illustrates a differential sense amplifier which is an exampleof sense amplifier circuits. The differential sense amplifier has inputterminals Vin(+) and Vin(−) and an output terminal Vout, and amplifies adifference between a potential of Vin(+) and a potential of Vin(−). Ifthe potential of Vin(+) is higher than the potential of Vin(−), theoutput from Vout is relatively high, whereas if the potential of Vin(+)is lower than the potential of Vin(−), the output from Vout isrelatively low. In the case where the differential sense amplifier isused for the reading circuit, one of the input terminals Vin(+) andVin(−) is connected to the terminal A, and the reference potential Vrefis supplied to the other of the input terminals Vin(+) and Vin(−).

FIG. 9C illustrates a latch sense amplifier which is an example of senseamplifier circuits. The latch sense amplifier has input/output terminalsV1 and V2 and input terminals for control signals Sp and Sn. First, thesignal Sp is set high and the signal Sn is set low, and a power supplypotential (Vdd) is interrupted. Then, potentials to be compared aresupplied to V1 and V2. After that, the signal Sp is set low and thesignal Sn is set high, and the power supply potential (Vdd) is supplied.If the potentials V1in and V2 in to be compared satisfy V1in>V2 in, theoutput from V1 is high and the output from V2 is low, whereas if thepotentials satisfy V1in<V2 in, the output from V1 is low and the outputfrom V2 is high. By utilizing such a relationship, the differencebetween V1in and V2 in can be amplified. In the case where the latchsense amplifier is used for the reading circuit, one of V1 and V2 isconnected to the terminal A and an output terminal through a switch, andthe reference potential Vref is supplied to the other of V1 and V2.

With the use of any of the transistors described in the aboveembodiments as a writing transistor of a memory cell in theabove-described semiconductor device which can be used as a memorydevice, the short-channel effect of the writing transistor can besuppressed, and miniaturization can be achieved. Accordingly, thesemiconductor device which can be used as a memory device can havehigher integration.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the structures, methods, and thelike described in the other embodiments.

Embodiment 5

In this embodiment, an example of application of a semiconductor deviceaccording to one embodiment of the disclosed invention will be describedwith reference to FIG. 10. Here, a central processing unit (CPU) isdescribed.

FIG. 10 illustrates an example of a block diagram of a CPU. A CPU 1101illustrated in FIG. 10 includes a timing control circuit 1102, aninstruction decoder 1103, a register array 1104, an address logic andbuffer circuit 1105, a data bus interface 1106, an arithmetic logic unit(ALU) 1107, an instruction register 1108, and the like.

These circuits are manufactured using any of the transistors describedin the above embodiments, an inverter circuit, a resistor, a capacitor,and the like. Because the transistors described in the above embodimentscan achieve an extremely small off-state current, a reduction in powerconsumption of the CPU 1101 can be realized. Furthermore, with the useof any of the transistors described in the above embodiments, theshort-channel effect of the transistor can be suppressed, andminiaturization can be achieved.

Circuits included in the CPU 1101 will be briefly described below. Thetiming control circuit 1102 receives instructions from the outside,converts the instructions into information for the inside, and transmitsthe information to another block. In addition, the timing controlcircuit 1102 gives directions such as reading and writing of memory datato the outside, according to internal operation. The instruction decoder1103 functions to convert instructions from the outside intoinstructions for the inside. The register array 1104 functions totemporarily store data. The address logic and buffer circuit 1105functions to specify the address of an external memory. The data businterface 1106 functions to take data in and out of an external memoryor a device such as a printer. The ALU 1107 functions to perform anoperation. The instruction register 1108 functions to temporarily storeinstructions. The CPU includes such a combination of circuits.

With the use of any of the transistors described in the aboveembodiments in at least part of the CPU 1101, the short-channel effectof the transistor can be suppressed, and miniaturization can beachieved. Thus, the CPU 1101 can have higher integration.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the structures, methods, and thelike described in the other embodiments.

Embodiment 6

In this embodiment, an example of application of a semiconductor deviceaccording to one embodiment of the disclosed invention will be describedwith reference to FIGS. 11A and 11B. Here, an example of a semiconductordevice having an image sensor function for reading information of anobject will be described. Note that in some circuit diagrams, “OS” iswritten beside a transistor in order to indicate that the transistorincludes an oxide semiconductor.

FIG. 11A illustrates an example of a semiconductor device having animage sensor function. FIG. 11A is an equivalent circuit diagram of aphotosensor, and FIG. 11B is a cross-sectional view of part of thephotosensor.

One electrode of a photodiode 1202 is electrically connected to aphotodiode reset signal line 1212, and the other electrode of thephotodiode 1202 is electrically connected to a gate electrode of atransistor 1204. One of a source electrode and a drain electrode of thetransistor 1204 is electrically connected to a photosensor referencesignal line 1218, and the other of the source electrode and the drainelectrode of the transistor 1204 is electrically connected to one of asource electrode and a drain electrode of a transistor 1206. A gateelectrode of the transistor 1206 is electrically connected to a gatesignal line 1214, and the other of the source electrode and the drainelectrode of the transistor 1206 is electrically connected to aphotosensor output signal line 1216.

Here, transistors including an oxide semiconductor are used as thetransistor 1204 and the transistor 1206 illustrated in FIG. 11A. Here,any of the transistors described in the above embodiments can be used asthe transistors including an oxide semiconductor. Because thetransistors described in the above embodiments can achieve an extremelysmall leakage current in an off state, the photodetection accuracy ofthe photosensor can be improved. Furthermore, with the use of any of thetransistors described in the above embodiments, the short-channel effectof the transistor can be suppressed, and miniaturization can beachieved. Thus, the area of the photodiode can be increased, and thephotodetection accuracy of the photosensor can be improved.

FIG. 11B is a cross-sectional view illustrating the photodiode 1202 andthe transistor 1204 in the photosensor. The photodiode 1202 functioningas a sensor and the transistor 1204 are provided over a substrate 1222having an insulating surface (a TFT substrate). A substrate 1224 isprovided over the photodiode 1202 and the transistor 1204 using anadhesive layer 1228. An insulating layer 1234, an interlayer insulatinglayer 1236, and an interlayer insulating layer 1238 are provided overthe transistor 1204.

A gate electrode layer 1240 is provided in the same layer as the gateelectrode of the transistor 1204 so as to be electrically connected tothe gate electrode. The gate electrode layer 1240 is electricallyconnected to an electrode layer 1242 provided over the interlayerinsulating layer 1236, through an opening formed in the insulating layer1234 and the interlayer insulating layer 1236. Because the photodiode1202 is formed over the electrode layer 1242, the photodiode 1202 andthe transistor 1204 are electrically connected to each other through thegate electrode layer 1240 and the electrode layer 1242.

The photodiode 1202 has a structure in which a first semiconductor layer1226 a, a second semiconductor layer 1226 b, and a third semiconductorlayer 1226 c are stacked in this order over the electrode layer 1242. Inother words, the first semiconductor layer 1226 a of the photodiode 1202is electrically connected to the electrode layer 1242. The thirdsemiconductor layer 1226 c of the photodiode 1202 is electricallyconnected to an electrode layer 1244 provided over the interlayerinsulating layer 1238.

Here, a PIN photodiode is given as an example, in which a semiconductorlayer having n-type conductivity as the first semiconductor layer 1226a, a high-resistance semiconductor layer (an i-type semiconductor layer)as the second semiconductor layer 1226 b, and a semiconductor layerhaving p-type conductivity as the third semiconductor layer 1226 c arestacked.

The first semiconductor layer 1226 a is an n-type semiconductor layerand is formed with an amorphous silicon film containing an impurityelement imparting n-type conductivity. The first semiconductor layer1226 a is formed by a plasma CVD method with the use of a semiconductorsource gas containing an impurity element belonging to Group 15 (such asphosphorus (P)). As the semiconductor source gas, silane (SiH₄) may beused. Alternatively, disilane (Si₂H₆), dichlorosilane (SiH₂Cl₂),trichlorosilane (SiHCl₃), silicon tetrachloride (SiCl₄), silicontetrafluoride (SiF₄), or the like may be used. Alternatively, anamorphous silicon film which does not contain an impurity element may beformed, and then, an impurity element may be introduced into theamorphous silicon film by a diffusion method or an ion implantationmethod. After the impurity element is introduced by an ion implantationmethod or the like, heating or the like may be conducted in order todiffuse the impurity element. In this case, as a method for forming theamorphous silicon film, an LPCVD method, a vapor deposition method, asputtering method, or the like may be used. The first semiconductorlayer 1226 a is preferably formed so as to have a thickness of 20 nm to200 nm.

The second semiconductor layer 1226 b is an i-type semiconductor layer(an intrinsic semiconductor layer) and is formed with an amorphoussilicon film. As the second semiconductor layer 1226 b, an amorphoussilicon film is formed by a plasma CVD method with use of asemiconductor source gas. As the semiconductor source gas, silane (SiH₄)may be used. Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or thelike may be used. The second semiconductor layer 1226 b mayalternatively be formed by an LPCVD method, a vapor deposition method, asputtering method, or the like. The second semiconductor layer 1226 b ispreferably formed so as to have a thickness of 200 nm to 1000 nm.

The third semiconductor layer 1226 c is a p-type semiconductor layer andis formed with an amorphous silicon film containing an impurity elementimparting p-type conductivity. The third semiconductor layer 1226 c isformed by a plasma CVD method with the use of a semiconductor source gascontaining an impurity element belonging to Group 13 (such as boron(B)). As the semiconductor source gas, silane (SiH₄) may be used.Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or the like may beused. Alternatively, an amorphous silicon film which does not contain animpurity element may be formed, and then, an impurity element may beintroduced into the amorphous silicon film by a diffusion method or anion implantation method. After the impurity element is introduced by anion implantation method or the like, heating or the like may beperformed in order to diffuse the impurity element. In this case, as amethod for forming the amorphous silicon film, an LPCVD method, a vapordeposition method, a sputtering method, or the like may be used. Thethird semiconductor layer 1226 c is preferably formed so as to have athickness of 10 nm to 50 nm.

The first semiconductor layer 1226 a, the second semiconductor layer1226 b, and the third semiconductor layer 1226 c are not necessarilyformed using an amorphous semiconductor, and they may be formed using apolycrystalline semiconductor or a microcrystalline semiconductor (or asemi-amorphous semiconductor (SAS)).

The microcrystalline semiconductor belongs to a metastable state whichis an intermediate state between an amorphous state and a single crystalstate according to Gibbs free energy. That is, the microcrystallinesemiconductor is a semiconductor having a third state which isthermodynamically stable and has a short range order and latticedistortion. In the microcrystalline semiconductor, columnar orneedle-like crystals grow in a normal direction with respect to asurface of a substrate. The Raman spectrum of microcrystalline silicon,which is a typical example of the microcrystalline semiconductor, isshifted to a smaller wavenumber region than 520 cm⁻¹ which representssingle crystal silicon. That is, the peak of the Raman spectrum ofmicrocrystalline silicon exists between 520 cm⁻¹ which represents singlecrystal silicon and 480 cm⁻¹ which represents amorphous silicon. Themicrocrystalline semiconductor includes at least 1 at. % of hydrogen orhalogen to terminate a dangling bond. Moreover, a rare gas element suchas helium, argon, krypton, or neon may be included to further promotelattice distortion, so that a favorable microcrystalline semiconductorfilm with enhanced stability can be obtained.

This microcrystalline semiconductor film can be formed by ahigh-frequency plasma CVD method with a frequency of several tens toseveral hundreds of megahertz or a microwave plasma CVD method with afrequency of 1 GHz or more. Typically, the microcrystallinesemiconductor film can be formed by using a gas obtained by diluting agas including silicon, such as SiH₄, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, orSiF₄, with hydrogen. Alternatively, the microcrystalline semiconductorfilm can be formed by using a gas including silicon which is dilutedwith hydrogen and one or more rare gas elements selected from helium,argon, krypton, and neon. In this case, the flow rate of hydrogen is set5 times to 200 times, preferably 50 times to 150 times, more preferably100 times, as high as that of a gas including silicon. Furthermore, agas including silicon may be mixed with a hydrocarbon gas such as CH₄ orC₂H₆, a gas including germanium gas such as GeH₄ or GeF₄, F₂, or thelike.

The mobility of holes generated by the photoelectric effect is lowerthan the mobility of electrons. Therefore, a PIN photodiode has bettercharacteristics when a surface on the p-type semiconductor layer side isused as a light-receiving plane. Here, an example where the photodiode1202 receives incident light 1230 from the substrate 1224 side andconverts it into electric signals is described. Further, light from aside on which the semiconductor layer having a conductivity typeopposite to that of the semiconductor layer on the light-receiving planeside is disturbance light; therefore, the electrode layer 1242 ispreferably formed using a light-blocking conductive film. Note that then-type semiconductor layer side may alternatively be a light-receivingplane.

When the incident light 1230 enters from the substrate 1224 side, theoxide semiconductor layer of the transistor 1204 can be shielded fromthe incident light 1230 by the gate electrode of the transistor 1204.

The insulating layer 1234, the interlayer insulating layer 1236, and theinterlayer insulating layer 1238 can be formed using an insulatingmaterial by a method such as a sputtering method, an SOG method, a spincoating method, a dip-coating method, a spray coating method, a screenprinting method, an offset printing method or a droplet discharge method(e.g., an inkjet method) depending on the material.

The insulating layer 1234 may be a single layer or stacked layers of aninorganic insulating material, with any of oxide insulating layers ornitride insulating layers such as a silicon oxide layer, a siliconoxynitride layer, a silicon nitride layer, a silicon nitride oxidelayer, an aluminum oxide layer, an aluminum oxynitride layer, analuminum nitride layer, or an aluminum nitride oxide layer. In addition,a high-quality insulating layer which is dense and has high withstandvoltage can be formed by a high-density plasma CVD method usingmicrowaves (2.45 GHz), which is preferable.

For a reduction of the surface roughness, an insulating layerfunctioning as a planarization insulating film is preferably used as theinterlayer insulating layers 1236 and 1238. The interlayer insulatinglayers 1236 and 1238 can be formed using an organic insulating materialhaving heat resistance such as a polyimide, an acrylic resin,benzocyclobutene-based resin, a polyamide, or an epoxy resin. Other thansuch organic insulating materials, it is possible to use a single layeror stacked layers of a low dielectric constant material (a low-kmaterial), a siloxane-based resin, phosphosilicate glass (PSG),borophosphosilicate glass (BPSG), or the like.

The photodiode 1202 can read information of an object by detecting theincident light 1230. Note that a light source such as a backlight can beused at the time of reading information of an object.

In the photosensor described above, any of the transistors described inthe above embodiments can be used as the transistor including an oxidesemiconductor. Because the transistors described in the aboveembodiments can achieve an extremely small leakage current in an offstate, the photodetection accuracy of the photosensor can be improved.Furthermore, with the use of any of the transistors described in theabove embodiments, the short-channel effect of the transistor can besuppressed, and miniaturization can be achieved. Thus, the area of thephotodiode can be increased, and the photodetection accuracy of thephotosensor can be improved.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the structures, methods, and thelike described in the other embodiments.

Embodiment 7

In this embodiment, the cases where any of the semiconductor devicesdescribed in the above embodiments is applied to electronic devices willbe described with reference to FIGS. 12A to 12F. The cases where any ofthe above-described semiconductor devices is applied to electronicdevices such as a computer, a mobile phone set (also referred to as amobile phone or a mobile phone device), a portable information terminal(including a portable game machine, an audio reproducing device, and thelike), a digital camera, a digital video camera, electronic paper, atelevision set (also referred to as a television or a televisionreceiver), and the like are described in this embodiment.

FIG. 12A illustrates a notebook personal computer, which includes ahousing 701, a housing 702, a display portion 703, a keyboard 704, andthe like. At least one of the housings 701 and 702 is provided with anyof the semiconductor devices described in the above embodiments.Therefore, a notebook personal computer which operates at high speed andconsumes less power, for example, can be realized.

FIG. 12B illustrates a portable information terminal (PDA). A main body711 is provided with a display portion 713, an external interface 715,operation buttons 714, and the like. Further, a stylus 712 for operationof the portable information terminal, or the like is provided. The mainbody 711 is provided with any of the semiconductor devices described inthe above embodiments. Therefore, a portable information terminal whichoperates at high speed and consumes less power, for example, can berealized.

FIG. 12C illustrates an electronic book 720 incorporating electronicpaper, which includes two housings, a housing 721 and a housing 723. Thehousing 721 and the housing 723 include a display portion 725 and adisplay portion 727, respectively. The housing 721 is connected to thehousing 723 by a hinge 737, so that the electronic book 720 can beopened and closed using the hinge 737 as an axis. In addition, thehousing 721 is provided with a power switch 731, operation keys 733, aspeaker 735, and the like. At least one of the housings 721 and 723 isprovided with any of the semiconductor devices described in the aboveembodiments. Therefore, an electronic book which operates at high speedand consumes less power, for example, can be realized.

FIG. 12D illustrates a mobile phone set, which includes two housings, ahousing 740 and a housing 741. The housings 740 and 741 in a state wherethey are developed as illustrated in FIG. 12D can be slid so that one islapped over the other. Therefore, the size of the mobile phone set canbe reduced, which makes the mobile phone set suitable for being carriedaround. The housing 741 includes a display panel 742, a speaker 743, amicrophone 744, a touch panel 745, a pointing device 746, a camera lens747, an external connection terminal 748, and the like. The housing 740includes a solar cell 749 for charging the mobile phone set, an externalmemory slot 750, and the like. An antenna is incorporated in the housing741. At least one of the housings 740 and 741 is provided with any ofthe semiconductor devices described in the above embodiments. Therefore,a mobile phone set which operates at high speed and consumes less power,for example, can be realized.

FIG. 12E illustrates a digital camera, which includes a main body 761, adisplay portion 767, an eyepiece 763, an operation switch 764, a displayportion 765, a battery 766, and the like. The main body 761 is providedwith any of the semiconductor devices described in the aboveembodiments. Therefore, a digital camera which operates at high speedand consumes less power, for example, can be realized.

FIG. 12F is a television set 770, which includes a housing 771, adisplay portion 773, a stand 775, and the like. The television set 770can be operated with a switch included in the housing 771 or with aremote controller 780. The housing 771 and the remote controller 780 areprovided with any of the semiconductor devices described in the aboveembodiments. Therefore, a television set which operates at high speedand consumes less power, for example, can be realized.

As described above, the electronic devices described in this embodimenteach include any of the semiconductor devices according to the aboveembodiments. Therefore, an electronic device whose operation speed isincreased and power consumption is reduced by miniaturization of thesemiconductor device can be obtained.

Example 1

In this example, results of examination, by computational simulation, ofthe effect of the disclosed invention in terms of suppression of ashort-channel effect will be described. Note that calculations (a firstcalculation and a second calculation) were carried out using a devicesimulator “Atlas” produced by Silvaco Data Systems Inc.

The first calculation was carried out using models illustrated in FIGS.13A and 13B. FIG. 13A illustrates a transistor having a structure inwhich there is a small difference in height between an upper surface ofan insulating layer and an upper surface of a source electrode and adrain electrode (hereinafter, Structure A), according to one embodimentof the disclosed invention. FIG. 13B illustrates a transistor having astructure in which there is no such difference in height (hereinafter,Structure B), as a comparative example. Note that a structure in whichan insulating layer 143 b does not cover a source electrode 142 a and adrain electrode 142 b as illustrated in FIG. 13A is employed asStructure A for simplicity of the calculation; there is not muchcomputational difference between the structure and a structure in whichthe insulating layer 143 b covers the source electrode 142 a and thedrain electrode 142 b.

In the calculation, the inclination angle θ of a side surface of aregion where the difference in height is made (hereinafter referred toas a projecting region) and the height h of the projecting region werevariables. Note that components denoted by reference numerals in FIGS.13A and 13B correspond to the components denoted by the same referencenumerals and described in the above embodiment. In addition, thecalculation models of this example do not include some components suchas the insulating layer 150, but this does not affect the result ofcalculation.

Other parameters used for the calculation are as follows.

-   -   Thickness of oxide semiconductor layer: 10 nm    -   Material of oxide semiconductor layer: In—Ga—Zn—O-based metal        oxide (band gap Eg: 3.15 eV, electron affinity (χ): 4.3 eV,        relative permittivity: 15, electron mobility: 10 cm²/Vs)    -   Thickness of gate insulating layer: 10 nm    -   Material of gate insulating layer: hafnium oxide (relative        permittivity: 15)    -   Material of source and drain electrodes: titanium nitride (work        function: 3.9 eV)    -   Material of gate electrode: tungsten (work function: 4.9 eV)

The results of the first calculation are shown in FIG. 14, FIG. 15, andFIG. 16. FIG. 14 shows the relationship between gate voltage VG (V) anddrain current ID (A); FIG. 15, the relationship between channel length L(nm) and threshold voltage Vth (V); and FIG. 16, the relationshipbetween channel length L (nm) and S value (V/dec). Note that each ofFIG. 14, FIG. 15, and FIG. 16 shows results obtained with differentinclination angles 0 of 45°, 60°, and 90° and heights h of 5 nm, 10 nm,and 20 nm.

The results in FIG. 14, FIG. 15, and FIG. 16 show that there is nosignificant difference between Structure A and Structure B when thechannel length L is more than 100 nm, whereas a negative shift inthreshold voltage Vth and an increase in S value are suppressed inStructure A when the channel length L is 100 nm or less. That is, ashort-channel effect can be suppressed in Structure A, in comparisonwith Structure B.

The second calculation was carried out using models illustrated in FIGS.17A and 17B. FIG. 17A illustrates Structure A, and FIG. 17B illustratesStructure B. A difference between the first and second calculations isthat the distance between the source electrode and the drain electrodeis defined as the channel length L in the first calculation, whereas thechannel length L in the second calculation is measured along theprojecting shape of the insulating layer. In other words, in FIG. 17A,the channel length L equals Ls+Lc+Ld. By defining the channel length Lin FIG. 17A in this manner, the effect of an increase in effective valueof the channel length L can be counteracted, and an effect resultingfrom the shape can be accurately observed.

FIG. 18 shows the relationship between gate voltage VG (V) and draincurrent ID (A) which is obtained from the results of the secondcalculation. Here, the inclination angle θ is fixed to 90°, and theheight h is set to 5 nm, 10 nm, and 20 nm FIG. 18 shows that a negativeshift in threshold voltage Vth is suppressed in the structure in whichthere is a small difference in height between the upper surface of theinsulating layer and the upper surface of the source and drain electrode(Structure A), owing to its shape. In other words, it can be understoodthat the shape suppresses a short-channel effect.

Note that it can be seen from the results of the first and secondcalculations that a short-channel effect is less easily caused as theheight h increases. However, if there is a large difference in height, adecrease in coverage may cause a disconnection of the oxidesemiconductor layer or the like. Thus, the difference in height is setto 30 nm or less, preferably 20 nm or less.

This application is based on Japanese Patent Application serial no.2010-051008 filed with Japan Patent Office on Mar. 8, 2010, the entirecontents of which are hereby incorporated by reference.

1. (canceled)
 2. A semiconductor device comprising: an insulating layer;a source electrode and a drain electrode embedded in the insulatinglayer; an oxide semiconductor layer over and in contact with theinsulating layer, the source electrode, and the drain electrode; a gateinsulating layer over the oxide semiconductor layer; and a gateelectrode over the gate insulating layer, wherein a difference in heightexists between an upper surface of the insulating layer and each of anupper surface of the source electrode and an upper surface of the drainelectrode.
 3. The semiconductor device according to claim 2, wherein theupper surface of the insulating layer, which is in contact with theoxide semiconductor layer, has a root-mean-square roughness of 1 nm orless.
 4. The semiconductor device according to claim 2, wherein thedifference in height between the upper surface of the insulating layerand each of the upper surface of the source electrode and the uppersurface of the drain electrode is 5 nm or more.
 5. The semiconductordevice according to claim 2, wherein the difference in height betweenthe upper surface of the insulating layer and each of the upper surfaceof the source electrode and the upper surface of the drain electrode is20 nm or less.
 6. The semiconductor device according to claim 2, whereinthe oxide semiconductor layer contains indium and zinc.
 7. Asemiconductor device comprising: an insulating layer; a source electrodeand a drain electrode embedded in the insulating layer; an oxidesemiconductor layer over and in contact with the insulating layer, thesource electrode, and the drain electrode; a gate insulating layer overthe oxide semiconductor layer; and a gate electrode over the gateinsulating layer, wherein a difference in height exists between an uppersurface of the insulating layer and each of an upper surface of thesource electrode and an upper surface of the drain electrode, andwherein the upper surface of the source electrode and the upper surfaceof the drain electrode are partly covered with the insulating layer. 8.The semiconductor device according to claim 7, wherein the upper surfaceof the insulating layer, which is in contact with the oxidesemiconductor layer, has a root-mean-square roughness of 1 nm or less.9. The semiconductor device according to claim 7, wherein the differencein height between the upper surface of the insulating layer and each ofthe upper surface of the source electrode and the upper surface of thedrain electrode is 5 nm or more.
 10. The semiconductor device accordingto claim 7, wherein the difference in height between the upper surfaceof the insulating layer and each of the upper surface of the sourceelectrode and the upper surface of the drain electrode is 20 nm or less.11. The semiconductor device according to claim 7, wherein the oxidesemiconductor layer contains indium and zinc.
 12. The semiconductordevice according to claim 7, wherein an end portion of each of thesource electrode and the drain electrode has a tapered shape.
 13. Asemiconductor device comprising: a first transistor; and a secondtransistor over the first transistor, wherein the first transistorcomprises: a first channel formation region; a first gate insulatinglayer over the first channel formation region; a first gate electrodeover the first gate insulating layer so as to overlap with the firstchannel formation region; and a first source electrode and a first drainelectrode electrically connected to the first channel formation region,wherein the second transistor comprises: a second source electrode and asecond drain electrode embedded in an insulating layer; a second channelformation region over and in contact with the insulating layer, thesecond source electrode, and the second drain electrode; a second gateinsulating layer over the second channel formation region; and a secondgate electrode over the second gate insulating layer, wherein the secondchannel formation region comprises an oxide semiconductor layer, andwherein a difference in height exists between an upper surface of theinsulating layer and each of an upper surface of the second sourceelectrode and an upper surface of the second drain electrode.
 14. Thesemiconductor device according to claim 13, wherein the upper surface ofthe insulating layer, which is in contact with the second channelformation region, has a root-mean-square roughness of 1 nm or less. 15.The semiconductor device according to claim 13, wherein the differencein height between the upper surface of the insulating layer and each ofthe upper surface of the second source electrode and the upper surfaceof the second drain electrode is 5 nm or more.
 16. The semiconductordevice according to claim 13, wherein the difference in height betweenthe upper surface of the insulating layer and each of the upper surfaceof the second source electrode and the upper surface of the second drainelectrode is 20 nm or less.
 17. The semiconductor device according toclaim 13, wherein the oxide semiconductor layer contains indium andzinc.
 18. The semiconductor device according to claim 13, wherein theupper surface of the second source electrode and the upper surface ofthe second drain electrode are partly covered with the insulating layer.19. The semiconductor device according to claim 13, wherein an endportion of each of the second source electrode and the second drainelectrode has a tapered shape.
 20. A memory device including thesemiconductor device according to claim
 13. 21. A CPU comprising thesemiconductor device according to claim 13.